893 ISSN 1023-1935, Russian Journal of Electrochemistry, 2018, Vol. 54, No. 11, pp. 893–901. © Pleiades Publishing, Ltd., 2018. Original Russian Text © M.A. Kamensky, S.N. Eliseeva, G. Láng, M. Ujvári, V.V. Kondratiev, 2018, published in Elektrokhimiya, 2018, Vol. 54, No. 10S, pp. S70–S80. Electrochemical Properties of Overoxidized Poly-3,4-Ethylenedioxythiophene M. A. Kamensky a , S. N. Eliseeva a, *, G. Láng b , M. Ujvári b , and V. V. Kondratiev a a St. Petersburg State University, Institute of Chemistry, St. Petersburg, 199034 Russia b Etvs Loránd University, Institute of Chemistry, Budapest, 1117 Hungary *e-mail: svetlana.eliseeva@spbu.ru Received July 10, 2017; in final form, April 10, 2018 Abstract—The properties of poly(3,4-ethylenedioxythiophene) (PEDOT) films were studied electrochemi- cally at high positive potentials (from –0.3 to 1.5 V relative to the Ag/AgCl electrode). A cyclic voltammetry (CV) study revealed the range of potentials (up to 1.3–1.5 V) where the cycling leads to significant changes in the electrochemical, structural, and morphological properties of the polymer film due to overoxidation. When the upper cycling potential E up exceeded 1.4 V, the anodic current significantly increased during the first cycle and then decreased, which suggests a loss of the electroactivity of the polymer and degradation of its properties. In the high-frequency region of the impedance spectra of the PEDOT films, a semicircle appears after overoxidation, which indicates a notable increase of the charge transfer resistance in the system, in contrast to the films subjected to potentiodymanic processing in a limited range of potentials from –0.3 to 1.3 V. The effect of overoxidation on the polymer morphology was studied by scanning electron microscopy. The chemical state of elements in the structure of the polymer film was determined by X-ray photoelectron spectroscopy. The obtained data indicate that –S=O groups formed at the thiophene sulfur in the polymer. Keywords: conducting polymers, poly-3,4-ethylenedioxythiophene, overoxidation, cyclic voltammetry, impedance spectroscopy, electrochemical properties, degradation DOI: 10.1134/S1023193518130219 INTRODUCTION The conductive organic polymers with conjugated bonds and composite materials based on them remain the subject of active fundamental and applied electro- chemical research [1–8]. One of the problems dis- cussed in studies on the electrochemical properties of conducting organic polymer films is the degradation of film properties, i.e., the gradual reduction of their electroactivity under certain conditions (for example, polymer synthesis conditions, varying the nature of electrolyte, the use of high positive potentials and definite pH values of aqueous electrolytes, etc.). The determination of the conditions of stable electrochem- ical activity of polymers is extremely important for their practical use in various electrochemical devices, in particular, for determining the range of potentials in which the electrochemical behavior of the polymer is stable. Poly-3,4-ethylenedioxythiophene (PEDOT), which is a representative of conducting organic poly- mers with conjugated bonds, has attracted the atten- tion of researchers because of its high chemical stabil- ity and electroactivity in organic and aqueous solu- tions of electrolytes [9]. In recent years, various composite functional materials were obtained based on PEDOT, which have found application in energy storage devices (batteries and supercapacitors), in electrocatalysis and electrical analysis, solar cells, and many other applications [10–28]. The wide use of PEDOT in various electrochemical devices necessi- tates studies on the ranges of potentials for its stable electroactive behavior to reveal the factors that deter- mine the degradation of its electrochemical properties under definite conditions. These studies will make it possible to further develop the materials and select conditions for their optimum use. The electrochemical properties of the PEDOT polymer in neutral and acidic aqueous electrolytes are usually quite stable in the potential range from –0.3 to +0.8 V relative to the silver chloride reference elec- trode (SCE). In this range of potentials, reversible p doping of PEDOT occurs with the formation of posi- tively charged fragments in the polymer structure with appreciable charge delocalization within a fragment of the polymer chain (polarons and bipolarons). When a certain critical value (denoted as E up for simplicity) of the positive potential of the electrode is exceeded, so-called “overoxidation” of the polymer