7002
J . Phys. Chem. 1988, 92, 7002-7006
Chronoamperometry of Polypyrrole: Migration of Counterions and Effect of Uncompensated Solutlon Resistance Chris D. Paulse and Peter G. Pickup* Department of Chemistry, Memorial Uniueristy of Newfoundland, St. John’s, Newfoundland, Canada A1 B 3x7 (Received: January 26, 1988; In Final Form: May 17, 1988) Chronoamperometric transients for polypyrrole films in aqueous and acetonitrile solutions have been analyzed according to the single-pore porous electrode model. In 1 M KCl(aq), the resulting ionic resistivity of 3.3 X lo3 Q cm for polypyrrole is in excellent agreement with a value obtained from dc conductivity measurements. Analysis of the chronoamperometric data using a diffusion model results in diffusion coefficients that are several orders of magnitude higher than those measured by independent methods. It is concluded that, following a potential step, the major mode of counterion transport through polypyrrole is by migration rather than diffusion. The effect of the uncompensated solution resistanceon the chronoamperometric transients is also discussed.
Introduction The kinetics of oxidation and reduction of conducting polymer films such as polypyrrole, polythiophene, and polyaniline have recently been attracting increasing interest.’-” This topic is of central importance to many of the applications of these materials, particularly their application in secondary batteries.I2-l5 It is also of fundamental importance to our understanding of the structure of solvent-swollen conducting polymers. This is illustrated by the fact that attempts to improve the porosity of conducting polymer films have often been assessed by their influence on the redox kinetics of the films.4”~9J0J5 The redox kinetics of conducting polymers have been investigated by ac impedance method^,'*^^^^^^ chronocoulometry,24*’a cyclic ~ o l t a m m e t r ychronoabs~rptometry,~.~ ~~~-~ and chronoamp e r ~ m e t r y .It~ is generally agreed that, under conditions where the polymer is in a conducting state, the rates of oxidation and reduction are limited by counterion motion within the polymer film. In ac experiments this motion is generally measured as an ionic resistance which reflects the rate of migration of counterions. In cyclic voltammetric experiments and potential step experiments, counterion motion has been treated as diffusion and quantified as a diffusion coefficient. Pickup and Osteryoung4 have pointed out that potential step data for polypyrrole are more appropriately treated by using a migration model (porous electrode model). Several other g r o ~ p s l have ~ ’ ~ also considered the contribution of migration to (1) Bull, R. A.; Fan, F.-R. F.; Bard, A. J. J . Electrochem. Soc. 1982, 129, 1009-1 01 5 . (2) Genies, E. M.; Bidan, G.;Diaz, A. F. J. Electroanal. Chem. 1983,149, 101-1 13. (3) Genies, E. M.; Pernaut, J. M. Synth. Met. 1984/85, IO, 117-129. (4) Pickup, P. G.;Osteryoung, R. A. J. Electroanal. Chem. 1985, 195, 271-288. ( 5 ) Osaka, T.; Naoi, K.: Sakai, H.; Ogano, S. J . Elecfrochem. SOC.1987, 134, 285-289. (6) Shimidzu. T.: Ohtani. A.: Iwda. T.: Honda. K. J. Electroanal. Chem. 1987,’ 224, 123-135. (7) Glarum, S. H.; Marshall, J. H. J . Electrochem. SOC.1987, 134, 142-147. (8) Tanguy, J.; Mermillicd, N.; Hoclet, M. J . Electrochem. Soc. 1987, 134, 795-802. (9) Marque, P.: Roncali, J.; Garnier, F. J . Electroanal. Chem. 1987, 218, 107-118. (10) Sundaresan, N. S.: Basak, S.;Pomerantz, M.; Reynolds, J. R. J. Chem. SOC.,Chem. Commun. 1987, 621-622. (1 1) Rubinstein, I.; Sabatani, E.; Rishpon, J. J. Electrochem. SOC.1987, 1 3 4 , 3078-3083. (12) Yamamoto, T.; Zama, M.; Hishinuma, M.; Yamamoto, A. J . Appl. Electrochem. 1987, 17, 607. (13) Mermillicd, N.; Tanguy, J.; Petiot, F. J . Electrochem. Soc. 1986,133, 1073-1079. (14) Mohammadi, A.; Inganas, 0.; Lundstrom, I. J. Electrochem. SOC. 1986, 133, 947-949. (15) Osaka, T.; Naoi, K.; Ogano, S.; Nakamura, S. J . Electrochem. SOC. 1987. 134. 2096-2102. (16) Yap, W. T.; Durst, R. A.; Blubaugh, E. A,; Blubaugh, D. D. J. Electroanal. Chem. 1983, 144, 69-75. (17) Buck, R. P. J. Electroanal. Chem. 1987, 219, 23-48.
0022-3654/88/2092-7002$01.50/0
charge transport through polymer films. Schlenoff and ChienZ0have assessed the importance of migration in potential step experiments on polypyrrole by comparing the diffusion rate for radioactive C10,- self-exchange with diffusion coefficients obtained from potential step data by using a diffusion modeL2 They concluded that migration effects are unimportant. However, we will show that this conclusion is not justified and that the self-exchange data” actually support the migration model rather than the diffusion model for potential step experiments on polypyrrole. In this paper chronoamperometric data for partial reduction and reoxidation of conducting polypyrrole are analyzed by using the single-pore model for a porous electrode (migration model).*’ The resulting ionic resistivities of polypyrrole in various media are in agreement with independent ionic conductivity measurements on the same films and with ion mobility data from the literature. Treatment of the same data using the diffusion model yields diffusion coefficients that are too high by several orders of magnitude. These results clearly show that the kinetics of polypyrrole electrochemistry are limited by counterion migration rather than diffusion. We also address the problem of uncompensated solution resistance in potential step experiments on conducting polymers. Rubinstein et al.” have shown that the redox kinetics of thin polyaniline films can be limited solely by the uncompensated solution resistance. However, uncompensated solution resistance has only been discussed in one publication on potential step experiments on conducting polymer^.^ We show here that uncompensated solution resistance can cause significant errors in film ionic resistances measured by chronoamperometry and chronocoulometry.
Experimental Section Cells and Electrodes. Cyclic voltammetry and chronoamperometry experiments were conducted in three-compartment glass cells at 23 f 2 O C under an argon atmosphere. Working electrodes were polished (0.3-pm alumina) Pt disks sealed in glass (4.5 X low3cm2) or PTFE (0.458 cm2; Pine Instruments). The counter electrode was a Pt wire and SSCE (Fisher SCE containing saturated NaC1) reference electrodes were used. All potentials are reported relative to the SSCE. Uncompensated solution resistances (R,)were measured by an ac impedance technique using polypyrrole coated electrodes at +0.2 V. Rs was obtained as the high-frequency real-axis intercept of the complex plane impedance plot.22 (18) Lange, R.; Doblhofer, K. J. Electroanal. Chem. 1987, 237, 13-26. (19) Ashley, K.: Pons, S. Electrochemical Society Meeting, Philadelphia: Electrochemical Society: Pennington, NJ, 1987; p 515. (20) Schlenoff, J. B.; Chien, J. C. W. J . Am. Chem. SOC.1987, 109, 6269-6274. (21) De Levie, R. In Advances in Electrochemistry and Electrochemical Engineering, Delahay, P., Tobias, C. W., Eds.; Interscience: New York, 1967; Vol. 6.
0 1988 American Chemical Society
Chronoamperometry of Polypyrrole
The Journal of Physical Chemistry, Vol. 92, No. 24, 1988 7003
Electrolyte Solution
\
PTFE Block Gloss Slides Figure 1. Schematic diagram of the cell used for ionic resistance mea-
surements. A schematic diagram of the cell used for ionic conductivity measurements is shown in Figure 1.23 Two PTFE blocks with cavities are bolted together around two glass slides with holes, which sandwich the polypyrrole film. SSCE reference electrodes are positioned either side of the polypyrrole film, and the steady-state potential difference between them, induced by a constant current (10 PA-1 mA) between the Pt electrodes, is measured by using an Orion Research 601 Digital Ionanalyser. In an alternative experiment, the total resistance of the cell between the two 3.1-cm2 platinized Pt electrodes is measured by using a Beckman RC-18 Conductivity Bridge at 3 KHz. The potential of the polypyrrole film could not be controlled in these experiments. The rest potential of the polymer under these conditions is close to +0.2 V, which is within the range covered by the potential step experiments. Polypyrrole Growth and Removal from the Electrode. Polypyrrole films were grown at constant current (0.4-1.0 mA cm-2) from 0.5 M pyrrole (Aldrich; purified on a dry alumina column) in acetonitrile (Aldrich; HPLC grade) containing 0.1 or 1.O M tetraethylammonium perchlorate (TEAP; Fluka). A charge density of 0.24 C cm-2 is assumed to yield a 1.0-rm-thick film.' Polypyrrole films were removed from the 0.458-cm2 electrode for use in the ionic resistance measurements using transparent adhesive tape. A 6.5-mm-diameter hole was punched into the tape before use. This hole is smaller than the polypyrrole film (7.6-mm diameter) but larger than the holes in the glass slides of the conductivity cell (3.5-mm diameter). Therefore, the section of the polypyrrole whose conductivity is measured is never exposed to the adhesive tape. The polypyrrole can easily be removed from the tape by a jet of acetone. In some cases the polypyrrole film spontaneously peeled from the electrode on drying. Equipment. Electrochemical experiments were performed by using a Hokuto Denko HA-301 potentiostat and HB-104 function generator with a BBC MDL780 X-Y recorder. Chronoamperometric data were collected and analyzed by using a Tron microcomputer with a Data Translation DT2801 ADC/DAC card. A PAR-122 lock-in amplifier was used for the ac impedance measurements.
Results Cyclic Voltammetry. A typical cyclic voltammogram of a polypyrrole film in 0.1 M TEAP/acetonitrile is shown in Figure 2. The anodic and cathodic peaks at -0.15 and -0.58 V, respectively, are due to a redox process which changes the electronicz4and ionicz3 conductivity of the polymer. At potentials negative of these peaks, polypyrrole has substantially reduced electronic and ionic conductivity. At more positive potentials, the electronic conductivity reaches an almost constant value of ca. (22) Bard, A. J.; Faulkner, L . R. Electrochemical MethodsFundamentals and Applications; Wiley: New York, 1980. (23) Burgmayer, P.; Murray, R.W. J . Phys. Chem. 1984,88,2515-2521. (24)Feldman, B. J.; Burgmayer, P.; Murray, R. W. J . Am. Chem. SOC. 1985,IO?,a12-aia.
-ll--P=+ Potential (vs S S C/EV)
Figure 2. Cyclic voltammogram of a 3.5-pm-thick polypyrrole film in 0.1 M TEAP/CH,CN. Scan speed = 10 mV/s.
0
0.5
time / s
Figure 3. Current vs time transient for a +0.2 to +0.4 V potential step on a 3.5-pm polypyrrole film in 0.1 M TEAP/CH3CN. The solid line is the experimental data; the dashed line is theoretical (eq 1).
10 0-l cm-'. It can been from Figure 2 that the capacitance of the polymer also becomes almost constant. Thus the oxidized form of polypyrrole is similar to a porous metal.'~4~25 Potential Step Chronoamperometry. Porous Electrode Model. The electrochemical behavior of polypyrrole is simplest at potentials between +0.1 and + O S V. In this range, the capacitance, electronic conductivity, and ionic conductivity of the polymer are all approximately constant. This is confirmed by that fact that there are no significant differences between current vs time transients for anodic and cathodic potential steps in this potential range. Potential steps used in this work were therefore confined to this range, and no distinction is made between anodic and cathodic potential steps. The potential steps used were between +0.2 and +0.4 V in acetonitrile and between +O. 1 and +0.3 V in water. A current vs time plot for a 0.2-0.4 V potential step on a 3.5-p-thick polypyrrole film is shown in Figure 3 (solid line). Since polypyrrole appears to be similar to a porous metal in this potential region, the most appropriate model for the treatment of these data is the single-pore model for a porous electrode.*' In this model the charging rate of the electrode is limited by migration of ions down a cylindrical pore. With this model a polypyrrole film is and a capacitance (C,) characterized by an ionic resistance (RF) which are coupled to form a finite transmission line. A complete description of the experiment also requires consideration of the which appears in series uncompensated solution resistance (Rs), with the transmission line representing the film. Equation 1 describes the complete current transient for potentiostatic charging (25)Feldberg, S. W. J . Am. Chem. Soc. 1984,106, 4611-4674.
Paulse and Pickup
7004 The Journal of Physical Chemistry, Vol. 92, No. 24, 1988
TABLE I: Ionic Resistivities (PF) of Polypyrrole FUms from Chronoamperometry and dc Resistance Measurements pF/104 0 crn chronodc thickness/pm amperometry resistance film
electrolyte soh 0.1 M TEAP/CH,CN I
I -5
\ I
1
0.1 M KCl(aq) 1.0 M KCl(aq)
05
10
time / s Figure 4. The In (current) vs time plot for data from Figure 3.
of a finite thickness porous electrode through an uncompensated solution resistance according to this In eq 1 AI3 is the iRp _ --
2.5 3.5 4.9 6.7 7.9 12 4.9 4.4
4.7 3.1 4.0 4.1 4.0 3.2 2.2 1.8 2.0 1.8 0.86
3.3 4.4 5.7 11 17 17 25 27
I
0
d
0.26 0.32 0.43 0.24
0.36 0.44 0.32
sin (m) exp(-m2t/T) 2F(1
+ p ) sin (m) + pm cos (m)
(1)
magnitude of the potential step, 7 = RFCF, p = R$RF, and values of m are the positive roots of m tan (m) = l/p. Analysis of experimental data according to this equation can easily be accomplished if it is realized that the second and subsequent terms in the summation are only significant at short times. Thus a plot of In i vs t is linear for most of the current transient (Figure 4). The slope and intercept of the linear portion of this plot allow the calculation of RF and Rs if CF is known. CFis obtained from the integral of the complete current transient. For the data shown in Figures 3 and 4, RF and Rs were found to be 2.4 X lo3 and 6.1 X lo2 Q, respectively. The solution resistance is in excellent agreement with a value of 6.2 X lo2 Q measured by ac impedance. The film resistance corresponds to an ionic resistivity (PF) of 3.1 X 104 Q cm. A comparison of the theoretical and experimental curves is shown in Figure 3. The excellent fit clearly supports the validity of the model. Results for a number of other polypyrrole films in both acetonitrile and water are presented in Table I. It should be noted that the use of the intercept and slope of the linear portion of In i vs t plots to obtain RFand Rsoften leads to negative values for Rs, especially if R F / Ris~greater than about 4. This is due to the sensitivity of the result to small errors in the intercept. It is therefore generally preferable to obtain Rs from an independent measurement (e.g., ac impedance). RFcan then be determined from the slope of the In i vs t plot. Most of the data in Table I were obtained in this way. The analysis of 15 polypyrrole films in 0.10 M TEAP/acetonitrile by chronoamperometry using eq 1 has yielded an average film ionic resistivity (pF) of (4.1 f 0.8) X lo4 Q cm. This value appears to be independent of film thickness in the range of 2-12 pm. In aqueous KCl, the ionic resistivity of polypyrrole is lower and shows some dependence upon film thickness. It is almost constant a t 3 X lo3 51 cm for films thicker than 10 pm but increases for thinner films. A possible explanation for this is that the first few microns of a polypyrrole film form with a more compact structure than the bulk.15 However, the absence of a similar trend in acetonitrile does not support this suggestion. We will assume that the results for films thicker than 10 pm are more representative of bulk polypyrrole. The negligible differences between results in 0.1 and 1 M KCl suggest that the ionic resistivity of polypyrrole is independent of the electrolyte concentration in the bathing solution. This is reasonable considering that the counterion concentration within oxidized polypyrrole (ca. 2 M; see below) is significantly greater
I
I
5
1
IO
time-/2/ s-112 Figure 5. Current vs time-'/* plot for data from Figure 3.
than the bulk electrolyte concentration. The concentration of nonexchange electrolyte in the pores will be significantly less than in the bulk solution because of Donnan exclusion. Thus, even in 1 M KCl, the nonexchange electrolyte contribution to the ionic conductivity will be significantly smaller than the counterion contribution. Effect of Uncompensated Solution Resistance ( R s ) . In the above treatment of chronoamperometric data for polypyrrole, the effect of Rs was taken into account. However, the common method for treatment of such data involves the use of a current vs t-1/2 plot in which the effect of Rs appears as a curvature at short time which is generally ignored. In this section we assess the validity of the i vs t-1/2 plot for the analysis of chronoamperometric data for a porous electrode. For the porous electrode model the potentiostatic current transient for an infinitely thick electrode in the absence of a solution resistance is4s2' i = AAE(aC/?rpFt)'/2
(2)
where aC is the double-layer capacitance of 1 cm3 of polymer. The current transient shown in Figure 3 is plotted in this form in Figure 5 . It can clearly be seen that the experimental data do not yield a linear i vs ?-I/* plot. Two factors are responsible for this curvature. At short times, the uncompensated solution resistance (ca. 600 Q) causes the experimental current to be significantly lower than the theoretical current (eq 2). At long (27) Johnson, A. M.; Newman, J. J . Electrochem. SOC.1971, 118,
( 2 6 ) Posey, F. A,; Morozumi, T.J. Electrochem. Soc. 1966,113, 176-184
5 10-5 17.
The Journal of Physical Chemistry, Vol. 92, No. 24, 1988 7005
Chronoamperometry of Polypyrrole
TABLE II: RF/Rs Values from i vs t-1/2Plots (Using Eq 2) of Simulated Chronoamperomehic Data (Eq 1) R F I RS
simulated 10 4 1
RF/RS
i vs c - ' / ~ plot 11.2 5.4 2.8
i vs PI2 simulated 0.1 0.01
plot 2.2 2.0
times, the finite thickness of the film leads to lower experimental currents. Equation 2 should be applicable to the central portion of Figure 5. The most reasonable slope to use is that of a tangent to the curve from the origin, as shown in the figure. The slope of this tangent yields a film resistivity of 4.0 X lo4 il cm, which is in reasonable agreement with the value of 3.1 X lo4 R cm obtained using eq 1. In this case the i vs t-'Iz method results in an error of +30%. The i vs d 2 method (eq 2) has been further tested on simulated data (from eq l), and some of the results are shown in Table 11. The relative error depends only on RF/Rs and increases as RF/Rs decreases. For RF/Rs > 10 the i vs t-'Iz plot is adequate (error < 12%). However, as RF approaches Rs, the error becomes considerable and, for RF < Rs, RF cannot be estimated from a i vs t-l/z plot. Clearly, very significant errors can arise from the analysis of i vs t-'I2 plots that are distorted by an uncompensated solution resistance. Similar errors have been obtained with chronocoulometric data. Diffusion Model. In the literature, potentiostatic data for polypyrrole have generally been treated according to a diffusion model in which the chronoamperometric curve for an infinitely thick film is described byZ6 i = nFADI/Zc/t'/Za'/z
(3)
where D is the diffusion coefficient for counterions and c is the change in concentration of oxidized sites within the polymer. This model neglects the effect of Rs. Comparison of the slope from the tangent in Figure 5 with the diffusion model (eq 3) yields a counterion diffusion coefficient (D) of 1.5 X 1O-' cm2 s-l. For 15 polypyrrole films in 0.1 M TEAP/acetonitrile an average diffusion coefficient of 1.2 X lo-' cm2 s-l was obtained. For the three thickest films in 1 M aqueous KCl the average diffusion coefficient was 1.1 X lod cmz s-l. Ionic Resistance Measurements. The validity of the porous electrode model can be tested if the ionic resistivity of polypyrrole can be measured by an independent method. We have used ac and dc ionic resistance measurements similar to those used by Burgmayer and MurrayaZ3The cell shown in Figure 1 was used for both types of experiment. In the dc method, SSCEs were positioned either side of the polymer film to measure the potential drop across the film caused by a constant current between the platinized Pt electrodes. With 1 M KCl(aq) as the electrolyte and with currents of 10 PA-1 mA, the potential difference across the film reached a constant value after a few seconds, indicating that polarization of the electrolyte solution was not ~ignificant.~~ The resulting potential difference was proportional to the applied current. Application of Ohm's law yields the resitance of the polypyrrole film plus the resistance of the solution between the two SSCEs. An experiment with no polymer film gave a solution resistance of 35 il which was subtracted from all results. Film resistances ranged from 35 to 115 Q ; thus the maximum potential drop across a film was 1 15 mV at 1 mA. Ionic resistivities measured in this way for several polypyrrole films are given in Table I. These results are in good agreement with those obtained by chronoamperometry for films of similar thickness in 1 M KCl (Table I). The validity of the porous electrode model is clearly supported. (28) Daurn, P.; Lenhard, J. R.;Rolison, D.; Murray, R. W. J. Am. Chem.
SOC.1980,102, 4649-4653.
(29) Tiravanti, G . J . Membr. Sci. 1981, 9, 229-243.
This experiment was also tried with 0.1 M TEAP/acetonitrile as the electrolyte. However, the potential drop across the film increased rapidly due to polarization of the electrolytez9and did not reach a constant value. Extrapolation to zero time29did not yield reasonable results, often producing a negative current intercept. An alternative method, in which an ac conductivity bridge is used to measure the resistance across the cell, was therefore used. A control experiment with no polypyrrole film was used to obtain the solution resistance (ca. 480 il for 0.1 M TEAP/acetonitrile). This is subtracted from the resistance in the presence of the polypyrrole film to yield the film resistance. However, in these experiments, polypyrrole films often appeared to have negligible resistance and in several experiments the resistance with the film was slightly less than the resistance with no film. Suspecting that the electronic conductivity of the polypyrrole was responsible for the negligible resistance of the polypyrrole in this experiment, we replaced the polypyrrole film with copper foil, which is an electronic conductor with no ionic conductivity. A resistance of 680 R was obtained. In a further experiment, glass slides with no holes were used in the cell and a Pt wire was used to connect the solution in the two halves of the cell. The resistance was 120 R with the wire connected and 40000 R with no wire. Clearly, this type of experiment can respond to electronic conductivity as well as ionic conductivity. Presumably, double-layer charging of the surface of the electronic conductor is responsible for the current flow in the absence of ionic conductivity. It was concluded that the ionic resistivity of polypyrrole could not be measured by using the ac method.
Discussion The main aim of this work is to discover whether a migration model or a diffusion model is most appropriate for the treatment of potentiostatic data for polypyrrole. This can best be achieved by comparing the parameters obtained from these two models with similar parameters obtained by independent methods. To do this we use eq 4 to relate ionic film resistivities (pF) to ion diffusion coefficients (D).z3 In eq 4 Cm is the concentration of ion-exchange D = RT/ppFCIE
(4)
sites within the polymer (ca. 4 X mol cm-3z3). In this work CIE has been estimated from the charge under a voltammogram for oxidation of the film to the appropriate potential (Q(E))by using CIE= Q(E)/FAd
(5)
where d is the thickness of the film. CIEincreases with increasing potential because the film contains more cationic sites at higher potentials. For the films used in this work, values of CIEobtained by using eq 5 ranged from 2.0 X mol cm-3 at +0.1 V in 1 M aqueous KCl to 3.0 X mol cm-3 a t +0.4 V in 0.1 M TEAP/acetonitrile. CIE values for the initial and final potentials were averaged to yield 2.3 X and 2.8 X mol cme3 for 1 M KCl and acetonitrile, respectively. Since eq 4 neglects the effect of the nonexchange electrolyte within the polymer, results obtained by using this equation will slightly overestimate the counterion diffusion coefficient. However, this does not affect the validity of our conclusions since the diffusion and migration models yield results that differ by nearly 2 orders of magnitude. The errors arising from the use of eq 4 are very unlikely to exceed a factor of 2. For polypyrrole in 0.1 M TEAP/acetonitrile the ionic resistivity of 4.1 X 1O4 Q cm from the porous electrode model corresponds cmz s-l, according to eq to a diffusion coefficient of 2.3 X 4. This is almost 2 orders of magnitude smaller than the value of 1.2 X lo-' cmz s-' from the diffusion model. Clearly the two models are incompatible. A summary of the results of this work, together with relevant data from the literature, is given in Table 111. The results for 1 M KC1 in Table 111are based only on the data for films thicker than 10 pm since thinner films exhibited a thickness-dependent resistivity. Our ionic conductivity data and the diffusion coefficient
7006 The Journal of Physical Chemistry, Vol. 92, No. 24, 1988
Paulse and Pickup
TABLE III: Counterion Diffmion Coefficients (D/cm3 s-l) in Polvovrrole this work
electrolyte soh 1 M KCl(aq) 1 M LiClO,(aq) 0.1 M TEAP/CH3CN 1 M LiCI04/CH3CN a
migration model
diffusion model
3.5 x 10-8
1.1 x 10-6
2.3 x 10-9
1.2 x 10-7
ionic" resistance 3.3 x 10-8
ref 23 >2 x 10-86 1.2 x 10%"
ref 20
4.2 X 3.6
X
From ionic resistivity using eq 4. b D times partition coefficient.
data from the literature are also for films thicker than 10 pm. All data in Table I11 are for oxidized polypyrrole. The main evidence supporting the migration model (porous electrode model) is the dc ionic conductivity data for polypyrrole in 1 M KCl(aq). The average ionic resistivity of 3.5 X lo3 fl cm is in excellent agreement with the value of 3.3 X lo3 fl cm obtained from chronoamperometric data on films of similar thickness by using the migration model. However, this resistivity corresponds to a diffusion coefficient of 3.3 X cm2 s-l, which does not agree with the value of 1 . 1 X 10" cm2 s-l from analysis of the same chronoamperometric data using the diffusion model. The validity of the migration model when applied to polypyrrole is also supported by counterion diffusion coefficient data from the literature. Murray and B ~ r g m a y e rhave ~ ~ estimated the diffusion coefficient for C1- in polypyrrole in aqueous 1 M KCl to be >2 X cm2 s-' from permeation measurements and 1.2 X lom8cm2 s-l from conductivity experiments. We obtain values of 3.5 X and 1.1 X 10" cm2 s-I by chronoamperometry using the migration and diffusion models, respectively. Clearly the migration model is supported. It should be noted that Murray and Burgmayer's measurements differ from ours in that their polypyrrole films contained an embedded gold minigrid electrode. The agreement between the two results suggests that the presence of this minigrid does not significantly affect the transport properties of the polypyrrole. Schlenoff and ChienZ0have reported diffusion coefficients of 4.2 X 10-8 and 3.6 X cm2 8' for C10,- in polypyrrole in water and acetonitrile, respectively. These values agree reasonably well with our values from the migration model for C1- in water and C104- in acetonitrile, respectively (Table 111). They are in poor agreement, however, with our results from the diffusion model. Again, the porous electrode model is supported. This is opposite to Schlenoff and Chien's conclusion from a comparison with chronoamperometric results reported by Genies et aL2 This contradiction can be explained if the data reported by Genies et al. are carefully examined. These authors used a film thickness of just 0.1 pm, which would have an ionic resistance of ca. 1 R, according to our results. The uncompensated solution resistance was ca. 100 R (based on the initial current in Figure 9 of ref 2). Table I1 shows that the slope of a i vs t-1/2 plot would be 2 orders of magnitude too low under these conditions. A similar error would occur in chronocoulometry and chronoabsorptometry. In fact, with such a small RF/Rsratio, it would be difficult to obtain any kinetic data concerning the polymer.
tential step data using the diffusion model are too high by more than an order of magnitude. The small-amplitude chronoamperometric behavior of the oxidized form of polypyrrole is described very well by the single-pore model for a porous electrode (eq 1). By use of this model it has been shown that for thin films (
