Electrochemical behaviour of poly(3,4-ethylenedioxytiophene) modified glassy carbon electrodes after overoxidation − the influence of the substrate on the charge transfer resistance

Abstract

<p class="PaperAbstract">Time dependence of the electrochemical impedance of an overoxidized glassy carbon|poly(3,4-ethylenedioxytiophene) (PEDOT)|0.1 mol·dm^-3 sulfuric acid (aq.) elec­trode has been investigated. To follow the changes occurring at the film/substrate interface after the overoxidation procedure, successive impedance measurements were carried out. Although the system is intrinsically nonstationary, the charge transfer resis­tance (R_ct) cor<strong>­</strong>res<strong>­</strong>ponding to different time instants could be determined by using the so-called 4-dimensional analysis method. The same post-experimental mathematic­cal/ana­lytical procedure could be used also for the estimation of the charge transfer resistance corresponding to the time instant just after overoxidation of the PEDOT film. The increase of the charge transfer resistance of the overoxidized system with respect to that of the pristine electrode suggests that during overoxidation the electrochemical activity of the film decreases and the charge transfer process at the metal/film interface beco­mes more hindered. After the overoxidation procedure, when the electrode potential was held in the “stability region” (at E = 0.4 V vs. SSCE in the present case) the R_ct decre­ased continuously with experiment time to a value somewhat higher than that of the pristine electrode.<strong> </strong>By comparing the properties of the GC|PEDOT|0.1 M H₂SO₄ and the Au|PEDOT|0.1 M H₂SO₄ electrodes a possible mechanistic explanation for the observed behavior has been proposed. This is based on the assumption that in the case of the GC|PEDOT|0.1 M H₂SO₄ electrode two processes may occur simultaneously during the impedance measurements: (a) reduction of the oxidized surface of the GC substrate, including the reduction of the oxygen-containing surface functionalities and (b) read­sorption of the polymer chains (polymer chain ends) on the surface.<strong></strong></p>