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