New Directions and Challenges in Electrochemistry
Electrochemical impedance spectroscopy of oxidized poly(3,4-ethylenedioxythiophene) film electrodes in aqueous solutions

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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.

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], [2], [3], [4], electrochromic devices [5], [6], [7], [8], [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], [13], [14], [15]. However, aqueous solutions have also been used. For example, PEDOT has been electrosynthesized from aqueous solutions containing different types of doping anions, including ClO4 [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 polymer-modified 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], [20], [21], [22], [23], [24].

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.

Section snippets

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, three-electrode electrochemical cell. The working electrode was a Pt disc electrode (area=0.07 cm2) and the auxiliary electrode

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.

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 (τD), diffusional pseudocapacitance (CD) and the diffusion resistance (RD=τD/CD). The model implies that electron transfer at the Pt | PEDOT interface and electron

Acknowledgements

The authors thank M.Sc. students Tomas Asplund, Eeva Helander, and Sanna Häggström 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 Åbo Akademi Process Chemistry Group, a National Centre of Excellence.

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