The Effect of Oxidation Ratio of Conducting Polymer on Potential Stability of the Conducting Polymer-Coated Electrode
Authors: Kusakabe Emi , Nakamura Yui , Fukuyama Mao , Maeda Kohji , Yoshida Yumi 
Published: February 2020

[Introduction]

 Conducting polymer-coated electrode was proposed from the second half of the 1980s as a solid electrode used in the organic phase (Org), and it has been applied as an internal electrode of ion-selective electrodes without an internal solution [1]. Conducting polymer-coated electrode works as the transducer of an ionic conductivity and an electronic conductivity because it is less soluble in Org and it captures the ions into the polymer of Org with its redox reaction. In recent years, the conducting polymer-coated electrode has applied to not only potentiometry but also voltammetry [2]. In our laboratory, we confirmed that it was applicable as both a reference electrode and a counter electrode－a reference/counter electrode－ in Org [3] and developed a thin layer electrolysis flow cell by using this electrode [4]. In the thin layer electrolysis flow cell, coulometry based on ion transfer and condensation of ion were realized [4].

 Despite many ion selective electrodes and electrolysis cells with the conducting polymer-coated electrode have been reported, there were few practical use ever. This is because, the conducting polymer-coated electrode has the low stability of the electrode potential [5]. In considering the Nernst equation, the ratio of oxidized form and reduced form of conducting polymer is 1:1, in other words, the 50 % oxidized conducting polymer is expected to reduce the effect on the electrode potential even if oxidation ratio is changed [3]. However, most of reported electrodes are those utilizing 100 % oxidized conducting polymer [6]. In this study, the oxidation ratio of the conducting polymer was confirmed based on its absorption spectrum and the relationship between the oxidation ratio of conducting polymer and the electrolysis potential was examined in order to obtain the high stability of the electrode potential.

[Experimental] 

 As a conducting polymer-coated electrode, poly(3,4-ethylenedioxythiophene) (PEDOT) coated electrodes was prepared by electropolymerization with constant current in 1.2-dichloroethane (DCE) containing 3,4-ethylenedioxythiophene (EDOT), and a hydrophilic supporting electrolyte, bis(triphenylphosphoranylidene)ammonium perchlorate ( BTPPA+ClO4
-), on the indium tin oxide glass. The thickness of the PEDOT was controlled by varying the electropolymerization current. Then the counter ion of the PEDOT was changed from ClO4
- to tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB-) by repeating oxidation-reduction procedures in DCE containing BTPPA+TFPB-. Potential stability was measured by potentiometry in the same solution. In order to determine the oxidation ratio of the PEDOT, UV-vis spectrum of the PEDOT coated-ITO electrode was measured.

[Results and Discussion] 

 The result of the absorption spectroscopy showed the PEDOT had two absorbance peaks at 570 nm and 860 nm. The peaks at 570 nm and 860 nm were attributed to the oxidized form and the reduced form of the PEDOT, respectively, since the absorbance at 570 nm increase while that of 860 nm decrease with the oxidation of the PEDOT. In addition, the isosbestic point was found at 700 nm in the absorption spectrum of the PEDOT measured under various potential. The potential to prepare the 50 % oxidized PEDOT electrode was estimated from the relationship between the absorbance ratio of 570 nm and 700 nm and the applied potential, which showed a sigmoidal curve as expected from the Nernst equation. The 50 % oxidized PEDOT electrode prepared based on this result showed potential change of less than 10 mV for 200 hours.

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