Electrochemical impedance spectroscopy of oxidized poly(3,4-ethylenedioxythiophene) film electrodes in aqueous solutions

www.elsevier.nl/locate/jelechem Journal of Electroanalytical Chemistry 489 (2000) 17 – 27

New Directions and Challenges in Electrochemistry

Electrochemical impedance spectroscopy of oxidized poly(3,4-ethylenedioxythiophene) film electrodes in aqueous solutions Johan Bobacka *, Andrzej Lewenstam, Ari Ivaska Process Chemistry Group, c/o Centre for Process Analytical Chemistry and Sensor Technology (ProSens), A, bo Akademi Uni6ersity, Biskopsgatan 8, FIN-20500 A, bo-Turku, Finland Received 24 January 2000; received in revised form 25 May 2000; accepted 27 May 2000

Abstract The electrochemical properties of oxidized (p-doped) poly(3,4-ethylenedioxythiophene) (PEDOT) film electrodes in aqueous solutions were investigated by electrochemical impedance spectroscopy (EIS). PEDOT was electrochemically deposited on platinum from aqueous solutions containing 0.01 M 3,4-ethylenedioxythiophene (EDOT) and 0.1 M supporting electrolyte: KCl, NaCl or poly(sodium 4-styrenesulfonate) (NaPSS). Impedance spectra were obtained for Pt/PEDOT electrodes at dc potentials where PEDOT is in the oxidized (p-doped) state. Electrodes with PEDOT films of different thickness, containing different doping ions, were investigated in contact with different aqueous supporting electrolyte solutions. The EIS data were fitted to an equivalent electrical circuit in order to characterize the electrochemical properties of the Pt/PEDOT film electrodes. Best fits to the experimental impedance data were obtained for an equivalent circuit where the total bulk (redox) capacitance of the polymer film is composed of the diffusional pseudocapacitance in series with a second bulk capacitance. The results imply that the PEDOT film contains an excess of supporting electrolyte, which facilitates ion diffusion and gives rise to a large diffusional pseudocapacitance. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Ac impedance; Equivalent circuit; Conducting polymer; Poly(3,4-ethylenedioxythiophene); Capacitance; Diffusion

1. Introduction Poly(3,4-ethylenedioxythiophene) (PEDOT) belongs to a group of very stable conducting polymers that are potential candidates for many technical applications including antistatic coatings and solid electrolyte capacitors [1–4], electrochromic devices [5 – 9], biosensors [10] and all-solid-state ion sensors [11]. Electrochemical and spectroelectrochemical characterization of PEDOT has been performed usually for PEDOT in contact with organic solutions [9,12 – 15]. However, aqueous solutions have also been used. For example, PEDOT has been electrosynthesized from aqueous solutions con-

* Corresponding author. Fax: +358-2-2154479. E-mail address: [email protected] (J. Bobacka).

taining different types of doping anions, including ClO− 4 [6], dodecyl sulfate [16,17] and poly(styrene sulfonate) (PSS−) [6,9,10]. Previous studies have shown that PEDOT is electroactive in aqueous solutions [10,16,17] exhibiting a stability superior to that of polypyrrole [10]. Furthermore, ion diffusion in PEDOT contacted by a polymer electrolyte was about three orders of magnitude faster than for other conjugated polymers [6]. On the basis of these findings, it is of interest to study the electrochemistry of PEDOT in more detail by using electrochemical impedance spectroscopy (EIS), which is a powerful technique to study charge transfer, ion diffusion and capacitance of conducting polymermodified electrodes [18]. We have used EIS earlier to develop equivalent electrical circuits to describe the electrochemical properties of poly(3-octylthiophene) film electrodes in organic solutions [19–24].

0022-0728/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 0 0 ) 0 0 2 0 6 - 0

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In the present work, EIS was used to study the charge transfer, ion diffusion and capacitance of PEDOT films doped with small mobile anions (Cl−) or large immobile polyanions (PSS−) expected to result in PEDOT films with anion- and cation-exchange behavior, respectively. The PEDOT films were studied in contact with aqueous solutions containing different anions (Cl−, PSS−) and cations (K+, Na+). The good stability of PEDOT allows an accurate characterization of its electrochemical properties without any significant degradation of the material.

2. Experimental The monomer, 3,4-ethylenedioxythiophene (EDOT, \ 97%), was obtained from Bayer AG. Poly(sodium 4-styrenesulfonate) (NaPSS, molar mass=70 000) was obtained from Aldrich. All other chemicals were analytical reagent grade. Distilled, deionized water was used to prepare all solutions. Electrochemical polymerization and measurements were performed by using a one-compartment, threeelectrode electrochemical cell. The working electrode was a Pt disc electrode (area =0.07 cm2) and the auxiliary electrode was a glassy carbon rod. The reference electrode was a Ag AgCl KCl (3 M) electrode. All dc potentials (Edc) are referred to this reference electrode. The cell solution was initially purged with nitrogen, and all experiments were performed under a nitrogen atmosphere at room temperature (239 2°C). All electrochemical measurements were made using an Autolab general purpose electrochemical system and Autolab frequency response analyser system (AUT20.FRA2AUTOLAB, Eco Chemie, B.V., The Netherlands). Prior to polymerization, the Pt working electrode was polished with 0.3 mm alumina, rinsed with water and cleaned ultrasonically. PEDOT was deposited on the Pt electrode by galvanostatic electrochemical polymerization from a deaerated aqueous solution containing 0.01

M EDOT and 0.1 M supporting electrolyte: KCl, NaCl or poly(sodium 4-styrenesulfonate) (NaPSS). A constant current of 0.014 mA (0.2 mA cm − 2) was applied for different times (71–1071 s) to produce polymerization charges in the range 1–15 mC (14–214 mC cm − 2). These polymerization charges correspond to film thicknesses in the range of approximately 0.1–1.5 mm, assuming 2.25 electrons/monomer and a film density of 1 g cm − 3. The supporting electrolyte used in the electrosynthesis of PEDOT are given in parenthesis, i.e. Pt/PEDOT(KCl), Pt/PEDOT(NaCl) and Pt/ PEDOT(NaPSS). The Pt/PEDOT electrodes in aqueous KCl, NaCl or NaPSS solutions were studied by electrochemical impedance spectroscopy (EIS) at Edc in the range −0.4 to +0.4 V. The Pt/PEDOT electrodes were equilibrated for at least 2 min at each Edc before EIS was performed. The impedance spectra were recorded in a frequency range (100 kHz–10 mHz) wide enough to cover the processes of interest, by using a sinusoidal excitation signal (single sine) with an excitation amplitude (DEac) of 10 mV. The impedance spectra were then fitted to an equivalent electrical circuit by using the Autolab impedance analysis software.

3. Results and discussion

3.1. Electropolymerization Chronopotentiometric curves recorded during galvanostatic electropolymerization of EDOT (0.01 M) in 0.1 M NaCl and 0.1 M NaPSS at a current density of 0.2 mA cm − 2 are shown in Fig. 1. The choice of the current density was based on the results presented by Yamato et al. [10] concerning potentiostatic polymerization of EDOT at different potentials. As can be seen in Fig. 1, the electropolymerization occurs at a lower potential in 0.1 M NaPSS than in 0.1 M NaCl as the supporting electrolyte. Electropolymerization in 0.1 M KCl gave the same chronopotentiometric curve as in 0.1 M NaCl. These results show that the PSS− polyanion facilitates the polymerization of EDOT compared with Cl− as the doping anion. Sakmeche et al. [17] observed a low polymerization potential for EDOT also in the presence of sodium dodecyl sulfate, which was explained by strong electrostatic interactions between EDOT radicals and dodecyl sulfate anions. A similar mechanism may be responsible for the low polymerization potential for EDOT in the presence of PSS− polyanions.

3.2. Electrochemical impedance spectroscopy (EIS) Fig. 1. Chronopotentiometric curves recorded during galvanostatic electropolymerization of EDOT (0.01 M) in 0.1 M NaCl and 0.1 M NaPSS as supporting electrolyte. Current density = 0.2 mA cm − 2.

EIS was performed at several Edc values in the range −0.4 to 0.4 V where PEDOT is in the oxidized state.

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KCl, QCV was found to be even higher after EIS, especially for thin films. These results demonstrate that no significant degradation of PEDOT occurred during the EIS measurements. However, a comparison of the cyclic voltammograms of the Pt/PEDOT(NaCl) electrode in 0.1 M NaCl (Fig. 2(a), QCV = 0.35 mC) to that of the Pt/PEDOT(NaPSS) electrode in 0.1 M NaPSS (Fig. 2(b), QCV = 0.50 mC) shows that the anion used in polymerization (Cl− or PSS−) significantly influences QCV. The polymerization charge was the same (10 mC) for both films presented in Fig. 2. The differences in QCV can be related to different amounts (film thickness) of PEDOT or different electroactivity (doping level) of PEDOT (for a given polymerization charge) depending on the anion used in the electropolymerization. After changing the supporting electrolyte from 0.1 M NaCl to 0.1 M NaPSS, and vice versa, the Pt/PEDOT electrodes were conditioned by cyclic voltammetry in the new electrolyte (50 cycles between −0.5 and 0.5 V at a scan rate of 0.1 V s − 1) before EIS was performed. As expected, the cyclic voltammogram of Pt/PEDOT

Fig. 2. Cyclic voltammograms before and after EIS for (a) the Pt/PEDOT(NaCl) electrode in 0.1 M NaCl, and (b) the Pt/PEDOT(NaPSS) electrode in 0.1 M NaPSS. Polymerization charge =10 mC. Potential scan rate =0.1 V s − 1.

Most of the discussion in this work is focused on the EIS results obtained at Edc =0.2 V, representing a potential where PEDOT is in the oxidized form. Before and after EIS, the electrochemical properties of the Pt/PEDOT electrodes were checked by cyclic voltammetry in the potential range −0.5 to 0.5 V (scan rate = 0.1 V s − 1). In this potential range, the cyclic voltammograms of the Pt/PEDOT electrodes do not reveal any clear oxidation or reduction peaks, but merely a capacitive-like current, as shown in Figs. 2 and 3. The charge (QCV) was obtained by integration of the cyclic voltammograms in the potential range − 0.5 to 0.5 V: QCV = (Qa + Qc)/2

(1)

where Qa and Qc are the anodic and cathodic charges, respectively. The charge (QCV) of the voltammograms in Fig. 2 obtained before and after EIS differs by 0.6% for the Pt/PEDOT(NaCl) electrode (Fig. 2(a)) and only by 0.05% for the Pt/PEDOT(NaPSS) electrode (Fig. 2(b)). For the Pt/PEDOT(KCl) electrodes in 0.1 M

Fig. 3. Cyclic voltammograms for (a) the Pt/PEDOT(NaCl) electrode in 0.1 M NaCl (thick line) and 0.1 M NaPSS (thin line), and (b) the Pt/PEDOT(NaPSS) electrode in 0.1 M NaPSS (thick line) and 0.1 M NaCl (thin line). Polymerization charge =10 mC. Potential scan rate =0.1 V s − 1.

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shape of the impedance plot as the Pt/PEDOT(KCl) electrode. At the lowest frequencies studied (down to 0.01 Hz), the impedance plots deviate from the 90° line, forming the beginning of a semicircle, as shown in Fig. 5 for a thin PEDOT film (polymerization charge= 1 mC). The

Fig. 4. Impedance plots of the Pt/PEDOT(KCl) electrode at different concentrations of supporting electrolyte (KCl): ( ) 0.1 M; ( ) 0.025 M; () 0.00625 M. Polymerization charge = 8 mC. Frequency range= 0.3 Hz – 10 kHz. Edc = 0.2 V and DEac = 10 mV.

(NaCl) is dependent on the anion of the supporting electrolyte (Fig. 3(a)). However, as can be seen in Fig. 3(a), the polymer shows a relatively high electroactivity also in NaPSS in spite of the bulkiness of the PSS− polyanion. On the other hand, the cyclic voltammogram of Pt/PEDOT(NaPSS) is practically insensitive to the anion of the supporting electrolyte (Fig. 3(b)), in agreement with the cation-exchange behavior of this film. A final check in the original electrolyte after these experiments revealed that QCV of the Pt/PEDOT(NaCl) electrodes decreased by 10 – 15%, while that of the Pt/PEDOT(NaPSS) electrodes decreased by less than 2%. Thus, the Pt/PEDOT(NaPSS) electrodes are stable in both 0.1 M NaPSS and 0.1 M NaCl, while some decrease in the electroactivity occurs for the Pt/PEDOT(NaCl) electrodes in 0.1 M NaPSS solution. The capacitance of PEDOT films determined by cyclic voltammetry is discussed together with the EIS results in Section 3.2.5.

3.2.1. General impedance characteristics Typical impedance spectra (complex plane impedance plots) of the Pt/PEDOT(KCl) electrode in 0.1, 0.025 and 0.00625 M KCl are shown in Fig. 4. The impedance plots are dominated by a 90° capacitive line, which extends down to very low frequencies (0.01 Hz) for thick films of PEDOT, as shown by Bobacka [11]. At high frequencies, there is only a slight deviation from the capacitive line, indicating fast charge transfer at the metal polymer and polymer solution interfaces, as well as fast charge transport in the polymer bulk. As shown in Fig. 4, the high frequency intersection with the Z% axis depends strongly on the electrolyte concentration and is consequently determined mainly by the solution resistance and not by the ohmic resistance of the polymer film. Also, the Pt/PEDOT(NaCl) and Pt/ PEDOT(NaPSS) electrodes show the same general

Fig. 5. Impedance plots of the Pt/PEDOT(KCl) electrode in 0.1 M KCl solution: (a) influence of Edc (frequency range =0.01 Hz–10 kHz); (b) influence of oxygen/nitrogen at Edc = −0.2 V (frequency range = 0.03 Hz – 10 kHz); (c) magnification of (b). Polymerization charge =1 mC and DEac =10 mV.

J. Bobacka et al. / Journal of Electroanalytical Chemistry 489 (2000) 17–27

Fig. 6. Equivalent electrical circuit for the Pt/PEDOT electrodes. Rs = solution resistance, ZD = finite-length Warburg diffusion impedance [26] and Cd = bulk (electronic) capacitance. The ZD element is characterized by the diffusional time constant (tD) and the diffusional pseudocapacitance (CD), as shown in Eq. (2).

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ferent polymerization charges (1−15 mC) and doping ions (Cl− and PSS−) at Edc between − 0.4 and 0.4 V was x 2 : 3× 10 − 4. A comparison of experimental and calculated (fitted) EIS data in the form of impedance, admittance and Bode plots are shown in Fig. 7. As can be seen in Fig. 7, the equivalent circuit presented in Fig. 6 can be used as a model for the Pt/PEDOT electrodes when excluding the low frequency data points that deviate significantly from the 90° capacitive line. The model is composed of the solution resistance (Rs), the capacitance (Cd) and the ‘classical’ finite-length Warburg diffusion element (ZD). The ZD element is characterized by the diffusional time constant (tD), the diffusional pseudocapacitance (CD) and the diffusion resistance (RD = tD/CD), as described by Macdonald [26]:

process was seen most clearly for such a thin film of PEDOT, while it was hardly observable for thick films (polymerization charge= 50 mC) [11]. As shown in Fig. 5, this low frequency process depends on Edc (Fig. 5(a)) and on the oxygen content of the solution (Fig. 5(b)). However, at frequencies higher than ca. 10 Hz (depending on film thickness), the oxygen content of the solution becomes of minor importance and the impedance plot approaches a 90° line (Fig. 5(c)). Similar impedance behavior was observed on bare Pt. Therefore, the low frequency process observed for the Pt/PEDOT electrodes can be related to a redox reaction at the electrode in parallel with the doping process, most probably due to traces of oxygen in the solution. The appearance of the low frequency semi-circle in the impedance plots observed in the presence of oxygen (Fig. 5(c)) is in agreement with the theory, which takes into account both ion and electron transfer between the polymer and solution, as recently described by Vorotyntsev et al. [25]. The presence of a parallel redox process was indicated also by the results obtained by cyclic voltammetry. For example, in the case of thin films of PEDOT(KCl) (polymerization charge=1 mC), the charge obtained from cyclic voltammetry (QCV) was found to increase from about 0.03 to 0.15 mC when decreasing the scan rate from 0.2 to 0.01 V s − 1, indicating the presence of a slow faradaic redox reaction occuring in parallel with the redox (doping) reaction of PEDOT. However, redox reactions between the Pt/PEDOT electrode and redox couples in solution will be studied in detail in a separate paper and are therefore not treated in any detail here. The discussion below is focused on the impedance response in the frequency range where the parallel redox process is not observed.

where j= (− 1)1/2 and v= angular frequency=2pf, where f= frequency in Hz. The impedance response of ZD is equivalent to that of a finite-length open transmission line where the finite diffusion length causes the phase angle to shift from 45 to 90° [26]. The capacitance Cd is necessary in the model to account for the deviation from an ideal 45° diffusion line at high and intermediate frequencies and it improves the quality of the fit significantly. Careful examination of experimental and fitted data in the impedance, admittance and Bode plots showed a better agreement when ZD and Cd were connected in series (Fig. 6) compared with the same circuit where ZD and Cd were connected in parallel. As will be shown below, both CD and Cd are related to the polymer bulk. A more classical equivalent circuit without Cd was used recently as a model for PEDOT [11]. In that model, the total bulk redox capacitance of PEDOT was represented by the diffusional pseudocapacitance alone. In the present work, the model is developed further (Fig. 6) by considering the total bulk redox capacitance as two bulk capacitances (CD and Cd) in series, which was found to improve the fit to the experimental EIS data. Furthermore, the decomposition of the bulk capacitance of a mixed conductor into an ionic and electronic part was recently suggested by Jamnik and Maier [27].

3.2.2. Equi6alent circuit Several electrical circuits were initially tested by nonlinear least-squares fitting of the experimental impedance data. The equivalent circuit shown in Fig. 6 was found to give excellent fits down to frequencies including the low frequency vertical line corresponding to the low frequency bulk capacitance of the polymer film (Fig. 4). The average error (x 2) of the fits for 89 different impedance spectra of PEDOT films with dif-

3.2.3. Resistance The solution resistance Rs was found to be inversely proportional to the concentration of the supporting electrolyte, as expected (Table 1). Furthermore, Rs is practically independent of film thickness and Edc. The values of Rs for different film thicknesses (polymerization charge= 1–15 mC) and Edc between − 0.4 and 0.4 V are as follows: 1229 1 V (0.1 M KCl), 1439 3 V (0.1 M NaCl) and 275 96 V (0.1 M NaPSS). These experimental results allow us to describe Rs as the solution

ZD = (tD/CD) coth (jvtD)1/2/(jvtD)1/2

(2)

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resistance. Any possible contribution from the ohmic (electronic) resistance of the oxidized PEDOT film to Rs falls within the experimental uncertainty. Since the maximum frequency used in the EIS experiment was 100 kHz, the possibility that Rs includes a charge transfer resistance (Rct) in parallel with a double-layer capacitance (Cdl) with a small time constant (RctCdl B

10 − 5 s) cannot be excluded. However, Rs seems to be dominated by the resistance of the electrolyte solution. As observed for Rs, the diffusion resistance RD of the Pt/PEDOT(KCl) electrodes was also found to be inversely proportional to the concentration of the supporting electrolyte, as shown in Table 1. Furthermore, RD was found to be practically independent of film

Fig. 7. Immittance data of the Pt/PEDOT(KCl) electrode in 0.1 M KCl solution. Polymerization charge = 8 mC. Frequency range =3 Hz–10 kHz. Edc =0.2 V and DEac = 10 mV. (a) Impedance plot: () experimental, ( ) calculated; (b) admittance plot: () experimental, ( ) calculated; (c) bode plot: (, ) experimental, ( , ×) calculated; the calculated data were obtained by fitting of the experimental data to the model shown in Fig. 6 (x2 =2.8 ×10 − 4): Rs = 122 V, RD = 41 V, CD = 2.32 mF, Cd =0.237 mF.

Table 1 EIS results for the Pt/PEDOT(KCl) electrode (polymerization charge =8 mC) obtained by fitting experimental data (Edc =0.2 V) to the model in Fig. 6 [KCl]/M 0.1 0.025 0.00625 a

Rs/V 122 449 1720

tD/s

CD/mF

0.08 0.39 1.13

2.11 2.14 1.72

RD/V

a

38 180 654

RD =tD/CD. Ctot = (1/CD+1/Cd)−1. c CCV = capacitance obtained by cyclic voltammetry (n= 0.1 V s−1) using Eq. (4). b

Cd/mF 238 232 222

Ctot/mF b 214 209 197

CCV/mF c 223 245 226

J. Bobacka et al. / Journal of Electroanalytical Chemistry 489 (2000) 17–27 Table 2 Diffusional time constant (tD), diffusional pseudocapacitance (CD) and diffusion resistance (RD = tD/CD) for Pt/PEDOT(NaPSS) and Pt/PEDOT(NaCl) electrodes in 0.1 M NaCl or 0.1 M NaPSS solution obtained by fitting experimental EIS data (Edc = 0.2 V) to the model in Fig. 6 a Doping ion

Electrolyte

PSS− PSS− PSS− PSS− PSS− PSS− Cl− Cl− Cl− Cl− Cl− Cl−

NaCl NaCl NaCl NaPSS NaPSS NaPSS NaCl NaCl NaCl NaPSS NaPSS NaPSS

a

Q/mC 5 10 15 5 10 15 5 10 15 5 10 15

tD/s

CD/mF

0.12 0.18 0.34 0.16 0.25 0.52 0.07 0.14 0.15 0.14 0.15 0.15

2.78 4.47 6.29 2.40 3.91 5.45 1.51 3.52 4.19 0.34 0.55 0.65

RD/V 43 35 54 65 65 96 48 39 35 408 267 231

Q= polymerization charge.

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As shown in Fig. 8, the value of CD is an order of magnitude higher than that of Cd for the same film thickness. This means that the total capacitance (Ctot) is determined mainly by Cd under the experimental conditions used. In contrast to the resistances (Rs and RD), the capacitances CD and Cd depend only slightly on the supporting electrolyte concentration (KCl), as shown in Table 1. This indicates that the magnitude of the bulk capacitance of the PEDOT film is determined primarily by the number of electronic charge carriers (polarons, bipolarons) in the polymer in the case of freely mobile doping ions, as observed earlier also for poly(3-octylthiophene) [23,24]. However, at the lowest electrolyte concentration studied (6.25 mM) one can see that CD is more sensitive than Cd to the ion concentration in solution. This is in good agreement with the model relating CD to the diffusional pseudocapacitance, as shown in Fig. 6 and Eq. (2). The physical meanings of CD and Cd are tentatively given as the ionic and

thickness (polymerization charge= 1 – 15 mC, Edc = 0.2 V) when using 0.1 M KCl or 0.1 M NaCl as the supporting electrolyte: RD =40 97 V. In these supporting electrolytes with anions and cations of high mobility (Na+, K+, Cl−), the diffusion resistance RD of the Pt/PEDOT electrodes is significantly lower than the solution resistance (Rs), independent of the doping anion of the PEDOT films (Tables 1 and 2). However, in 0.1 M NaPSS solution the Pt/PEDOT(NaCl) electrode shows a significantly higher value of RD than the Pt/ PEDOT(NaPSS) electrode, as shown in Table 2. This can be explained by the fact that the bulky PSS− anion cannot readily enter the polymer film. On the other hand, when the PSS− anion has been incorporated in the film already in the polymerization process, the low diffusion resistance (RD) is caused by the mobility of the charge-compensating Na+ cation in the film. These results obtained by EIS are in good agreement with potentiometric measurements, where PEDOT films doped with Cl− were found to give an anionic potentiometric response to Cl−, while PEDOT films doped with PSS− gave a cationic response [11].

3.2.4. Capacitance The capacitances CD and Cd increase linearly with the polymerization charge of the PEDOT film, as shown in Fig. 8. Both CD and Cd can therefore be related to the bulk properties of the PEDOT film. Therefore, it is important to note that the total polymer bulk (redox) capacitance (Ctot) is given as the two bulk capacitances (CD and Cd) connected in series (Fig. 6), as follows: Ctot =(1/CD +1/Cd) − 1

(3)

Fig. 8. Capacitances (a) CD and (b) Cd as a function of polymerization charge for ( ) Pt/PEDOT(KCl) electrodes in 0.1 M KCl and () Pt/PEDOT(NaCl) electrodes in 0.1 M NaCl. Edc =0.2 V.

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Table 3 Capacitance of Pt/PEDOT(NaPSS) and Pt/PEDOT(NaCl) electrodes in 0.1 M NaCl or 0.1 M NaPSS solution a Doping ion

Electrolyte

PSS− PSS− PSS− PSS− PSS− PSS− Cl− Cl− Cl− Cl− Cl− Cl−

NaCl NaCl NaCl NaPSS NaPSS NaPSS NaCl NaCl NaCl NaPSS NaPSS NaPSS

Q/mC 5 10 15 5 10 15 5 10 15 5 10 15

Cd/mF 243 458 670 226 439 656 150 315 433 94 180 246

Ctot/mF 224 415 606 207 395 586 136 289 392 74 136 178

CCV/mF 274 495 711 271 489 708 183 366 498 152 291 411

a

Capacitances Cd and Ctot were obtained from EIS by fitting experimental data (Edc = 0.2 V) to the model in Fig. 6 and by using Eq. (1). Capacitance CCV was obtained from cyclic voltammetry using Eq. (4). Q= polymerization charge.

pared with the Pt/PEDOT(NaCl) electrode. These results are in good agreement with the cyclic voltammograms shown in Fig. 2. Table 2 shows further that the diffusional pseudocapacitance (CD) of the Pt/ PEDOT(NaCl) electrodes is almost an order of magnitude lower in 0.1 M NaPSS than in 0.1 M NaCl. This can be related to the restricted ion transport (high RD) of the PSS− polyanion in the PEDOT(NaCl) film. In agreement with the equivalent circuit model (Fig. 6), the diffusional pseudocapacitance (CD) is again more sensitive to ion transport than Cd. However, in the case of severely restricted ion transport, also Cd and thus Ctot decrease, and the reversible oxidation of PEDOT becomes partly limited by slow ion transport (Table 3).

3.2.5. Capacitance obtained by EIS and by cyclic 6oltammetry In the case of conducting polymers, the capacitance obtained by impedance measurements may differ from that obtained by cyclic voltammetry, as shown by Tanguy et al. [28]. This discrepancy was related to conformational changes occuring during oxidation (doping) of conducting polymers resulting in an amplitude-dependent (quasi-reversible) redox process, as described by Ren and Pickup [29]. For comparison purposes, the voltammetric capacitance (CCV) of the Pt/PEDOT electrode was calculated from the cyclic voltammograms as follows [28]: CCV = I/n

Fig. 9. Capacitance (CCV) obtained from cyclic voltammograms of Pt/PEDOT(KCl) electrodes in 0.1 M KCl solution at different scan rates (Edc =0.2 V). Polymerization charges: () 1 mC; () 2 mC; ( ) 4 mC; ( ) 8 mC. For comparison, capacitances (Ctot) obtained by impedance measurements (Edc = 0.2 V) are indicated in the figure by dashed lines and by numbers.

electronic (space charge) contributions to the total bulk capacitance Ctot, respectively [21,27]. Both CD and Cd are higher for PEDOT films doped with PSS− than for PEDOT films doped with Cl−, for the same polymerization charge, as shown in Tables 2 and 3. This can be related to the differences observed in the galvanostatic electropolymerization, where PSS− resulted in a lower polymerization potential than Cl− (Fig. 1). Therefore, one can expect fewer side reactions during the growth of the PEDOT(NaPSS) film compared with the PEDOT(NaCl) film. Fewer side reactions should also correlate with a higher electroactivity of the polymer film, as reflected in the higher values of CD and Cd for the Pt/PEDOT(NaPSS) electrode com-

(4)

where I= current and n= potential scan rate. The current (I) was obtained as the average of the anodic and cathodic currents at Edc = 0.2 V. The voltammetric capacitance values (CCV) obtained for the Pt/PEDOT(KCl) electrodes are shown in Fig. 9. As can be seen in Fig. 9, CCV depends on the scan rate and film thickness (polymerization charge) in a rather complicated manner. Interestingly, CCV seems to pass through a minimum, which is shifted to lower scan rates with increasing film thickness. The scan rates (0.01–0.2 V s − 1) used to calculate CCV in Fig. 9 correspond to ac frequencies in the range 0.2–3.5 Hz, for a 10 mV RMS ac perturbation [29]. In this frequency range, the impedance plots already start to deviate from a 90° capacitive line, i.e. the Pt/PEDOT electrode does not behave as an ideal capacitor, especially for thin PEDOT films. As discussed above, the reason for the deviation from an ideal capacitor is probably a slow parallel faradaic redox reaction between traces of oxygen in solution and the Pt/PEDOT electrode. Since cyclic voltammetry gives the total current, CCV may differ from Ctot determined by EIS, depending on the kinetics and mechanism of the parallel redox reaction and the potential scan rate. As can be seen in Fig. 9, a good agreement between CCV and Ctot for all film thicknesses studied is observed only at a relatively high

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potential scan rate (n= 0.1 V s − 1) corresponding to ac frequencies where the PEDOT film still resembles a capacitor, i.e. the parallel redox reaction does not contribute to the response. So, for the Pt/PEDOT(KCl) electrodes in 0.1 M KCl the ratio CCV/Ctot =0.96 –1.04, when calculating CCV from cyclic voltammograms recorded at a potential scan rate of 0.1 V s − 1. As can be calculated from Tables 1 and 3, for Pt/PEDOT electrodes in supporting electrolytes with counterions that are highly mobile in the polymer film (Na+, K+, Cl−) the ratio CCV/Ctot =1.0 – 1.4. However, for Pt/PEDOT(NaCl) electrodes in 0.1 M NaPSS, i.e. in the case of restricted ion transport in the polymer film, the ratio CCV/Ctot = 2.1–2.3 (Table 3). These results show that the difference between the capacitances obtained by cyclic voltammetry and EIS is also greatly influenced by the ion transport properties of the conducting polymer film.

3.2.6. Diffusion The diffusional time constant (tD) is related to the diffusion coefficient (D) and the diffusion length (L) as follows [26]: tD =L 2/D

(5)

According to Eq. (5), tD should be proportional to L 2 if D is constant. Assuming a uniform film growth and film morphology, tD is therefore expected to be proportional to the square of the polymerization charge. However, the EIS results show that tD is directly proportional or even independent of the polymerization charge (Table 2). Consequently, if Eq. (5) is used directly assuming that L =film thickness, it gives apparent D values that increase with film thickness. For the films studied here, the apparent diffusion coefficient varies by an order of magnitude (D : 10 − 8 – 10 − 7 cm2 s − 1) within the thickness range studied, roughly 0.1– 1.5 mm. An increase of the apparent diffusion coefficient with film thickness was also reported for polypyrrole [30,31]. These features indicate that the diffusion length does not coincide with the film thickness, e.g. as a result of a polymer film containing a significant amount of supporting electrolyte. This is supported by the strong dependence of RD and tD on the supporting electrolyte concentration (Table 1). Such electrochemical behavior of PEDOT can be explained by a fibrillar morphology, which was reported for PEDOT doped with PF− 6 by Kiebooms et al. [32]. Furthermore, PEDOT doped with PSS− (trade name Baytron-P from Bayer AG, Germany) can form a highly swollen polymer hydrogel with a very high effective surface area, as reported by Ghosh and Ingana¨s [33]. Such an open film structure filled with supporting electrolyte allows fast ion diffusion in the supporting electrolyte phase of the film and the actual ion diffusion length (L) inside the polymer film may be much smaller

25

than the film thickness, in good agreement with the EIS results described in this work.

3.2.7. Potential dependence of EIS parameters Most of the discussion above was focused on the EIS results obtained at Edc = 0.2 V as representative of oxidized (doped) PEDOT, although the equivalent circuit developed (Fig. 6) is applicable to the whole potential range studied (Edc = − 0.4 to 0.4 V). The EIS parameters do not show any strong potential dependence in the given potential range. However, the potential dependence of the EIS parameters of PEDOT films of different thickness shows some trends that seem to be statistically significant. The most pronounced potential dependence was observed for CD, as shown in Fig. 10(a). For PEDOT doped with PSS− (cation-exchanger), CD decreases with increasing Edc, which can be related to a lower electrolyte concentration in the film at more positive Edc as a result of cation-expulsion from PEDOT as the oxidation level of PEDOT increases (Fig. 10a). Consequently, the opposite potential dependence of CD is seen for PEDOT doped with Cl− (anion exchanger) in 0.1 M NaCl electrolyte solution, where the anion is mobile (Fig. 10a). PEDOT doped with Cl− and immersed in NaPSS (anion of low mobility), on the other hand, is a special case of restricted ion transfer between PEDOT and the electrolyte solution and CD tends to decrease with increasing Edc (Fig. 10(a)). For this PEDOT–electrolyte combination, RD increases significantly with increasing Edc in contrast to the other PEDOT–electrolyte combinations studied (Fig. 10(b)). Finally, the potential dependence of Cd and Ctot (Fig. 10(c)) resembles the shape of the cyclic voltammograms. Further electrochemical characterization of PEDOT in aqueous solution by EIS should be combined with other experimental techniques such as EQCM to study changes in mass or rigidity of the polymer film and spectroelectrochemistry to study changes in doping level of the polymer as a function of Edc. 4. Conclusions The impedance response of the Pt/PEDOT electrode is properly described by an equivalent electrical circuit, which is composed of the solution resistance (Rs) and the ‘classical’ finite-length Warburg diffusion impedance (ZD) in series with a second bulk capacitance (Cd). The ZD element is characterized by the diffusional time constant (tD), diffusional pseudocapacitance (CD) and the diffusion resistance (RD = tD/CD). The model implies that electron transfer at the Pt PEDOT interface and electron transport in the bulk of PEDOT, as well as ion transfer at the PEDOT solution interface are fast, compared with ion

26

J. Bobacka et al. / Journal of Electroanalytical Chemistry 489 (2000) 17–27

diffusion in the polymer film. In spite of this, the diffusion resistance (RD) is comparable to or even smaller than the solution resistance (Rs), and practically independent of film thickness. Furthermore, RD is proportional to the concentration of supporting electrolyte in the solution, suggesting that the polymer film contains an excess of supporting electrolyte, which facilitates ion transport in the film. Based on the model

developed, the total bulk (redox) capacitane (Ctot) of PEDOT can actually be represented as two bulk capacitances in series, i.e. Ctot = (1/CD + 1/Cd) − 1, where CD is the diffusional pseudocapacitance and Cd is tentatively assigned as the electronic contribution to the bulk redox capacitance of the polymer. The experimental results thus suggest that the PEDOT film has a very large diffusional pseudocapacitance (CD) resulting from incorporated electrolyte in series with Cd. The EIS results described in this work are in good agreement with a fibrillar morphology and open film structure of PEDOT.

Acknowledgements The authors thank M.Sc. students Tomas Asplund, Eeva Helander, and Sanna Ha¨ggstro¨m for experimental assistance. Financial support from the National Technology Agency (TEKES) and Labsystems, Clinical Laboratory Division is gratefully acknowledged. This work has been supported by the Academy of Finland as a part of the A, bo Akademi Process Chemistry Group, a National Centre of Excellence.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

Fig. 10. Potential dependence of (a) CD, (b) RD, and (c) Ctot for Pt/PEDOT(NaPSS) in () 0.1 M NaCl and () 0.1 M NaPSS, and Pt/PEDOT(NaCl) in ( ) 0.1 M NaCl and ( ) 0.1 M NaPSS.

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