E. V. Ovsyannikova

Electrochemical and photoelectrochemical behaviour of polythiophenes in non-aqueous solutions - Part 2. The effect of charge trapping

Abstract The irreversibility of the doping–undoping processes and its effect on the electrochemical and photoelectrochemical behaviour of conducting polymers was studied for a series of polythiophene derivatives: polybithiophene, poly-3-methylthiophene and poly-3-phenylthiophene. For the three polymers it was confirmed that both anodic and cathodic undoping processes involve a considerable amount of residual charge trapped in the polymer phase. The nature of charge trapping for all the polymers was related to the incompleteness of the undoping processes rather than the chemical modification of the polymer, although the latter was also shown to occur. It was demonstrated that the trapped charge can be removed not only electrochemically (at the beginning of the polymer doping process of the opposite sign), as has been shown earlier, but also photoelectrochemically, by illuminating the polymer in solution at appropriate electrode potentials where the cathodic photocurrents are observed. The latter fact suggests that the photoelectrochemical undoping of these polymers observed by us earlier is in fact related to the photoassisted removal of the trapped charge.

Dynamic impedance measurements on a thin-film poly-3-methylthiophene electrode: Memory effects and space charge formation

Abstract A new procedure for measuring electrochemical impedance spectra directly during potential cycling is presented. The method consists in taking a series of impedance-potential curves at different potential modulation frequencies. Then the impedance values at desired electrode potentials are sampled, and corresponding dynamic impedance spectra are reconstructed. The method was applied to a thin-film poly-3-methylthiophene/vbPt (P3MT/vbPt) electrode in an acetonitrile solution. The results suggested that undoping of the polymer at negative potentials gave rise to development of a space-charge region within the film that hindered the polymer doping process unless flat-band conditions were reached. Such behavior could account for a well-known memory effect observed in conducting polymers. Moreover, our data provide further evidence that illumination of a P3MT electrode at negative potentials results in photoelectrochemical undoping of the polymer.

Determination of the surface potentials of solvents

Abstract A method for evaluation of the surface potential at the solvent-gas interface has been developed. It is based on the validity of the Born equation for the electrostatic part of the Gibbs solvation energy of large symmetric compact ions such as metallocenes, a result which was obtained earlier. The surface potential was found as the difference of the real solvation energy of the ferricinium cation and its chemical solvation energy calculated according to the Born equation. The values of surface potentials for six different solvents were obtained, and the data are compared with the results of other methods described in the literature.

Electrochemical and photoelectrochemical behaviour of poly-3-methylthiophene and polybithiophene in non-aqueous solutions: 1. Dark and light processes in systems with and without cathodic doping

The electrochemical as well as the photoelectrochemical behaviour of poly-3-methylthiophene and polybithiophene in systems with and without the possibility of cathodic doping was studied. Using electrochemical impedance measurements it was demonstrated that in both types of systems the photoelectrochemical processes on such polymers involved photoelectrochemical undoping of the polymer itself as well as the photoelectrochemical reduction of a component of an external redox couple in the solution. In systems with cathodic doping, in addition to these two processes, photoassisted cathodic doping of the polymer is observed. The model of photoelectrochemical processes on polymer electrodes is discussed.

Mechanism of cathodic reactions at electron-conducting polymer films: electroreduction of chloranil and tetracyanoquinodimethane at a poly-3-methylthiophene-coated glassy carbon electrode

Abstract The mechanism of the reversible one-electron reduction of tetracyanoquinodimethane (TCNQ), chloranil (CA) and o -dicarbollyl iron complex ( o -dCFe) at a poly-3-methylthiophene-coated glassy carbon electrode in acetonitrile has been studied. Values of the formal potentials of these substances are shifted in the negative direction with respect to the polymer film by 0.54 V, 0.74 V and 1.10 V respectively, corresponding to a decrease in the mobile charge carrier concentration in the polymer film. Reduction currents for both TCNQ and CA are lower than the diffusion-limiting currents at a bare electrode. They are only slightly lower for TCNQ, but for CA they are considerably less than the diffusion currents (a “kinetic limiting current” regime). The behaviour of the polymer film was interpreted in the framework of a p-type semiconductor model including equilibrium and non-equilibrium potential distributions across the metal + semiconductor + solution system. Reduction of o -dCFe was not observed at all because of its highly negative formal potential; hence its electrode behaviour is similar to that of an insulator.

Redox Conversions of Poly(3,4-ethylenedioxythiophene) and Its Copolymer with Bithiophene in Aprotic Media of Different Donor Capability

Electrochemical oxidation and reduction of poly(3,4-ethylenedioxythiophene) in solvents of different donor capabilities is studied by cyclic voltammetry. The reversibility of the anodic process is dependent on the solvent viscosity. The polymers anodic oxidation is presumably limited by the rate of removal of solvent molecules released during the desolvation of anions-dopants of the polymer matrix. The reversibility of the polymers cathodic reduction improves with increasing donor capability of the solvent. A residual charge of both signs discovered in the polymer points to its chemical heterogeneity. An in-principle feasibility of copolymerization of 3,4-ethylenedioxythiophene and bithiophene is shown.

Anodic Doping of Electropolymerized Copper 2,9,16,23-Tetraaminophthalocyanine

Anodic polymerization of 2,9,16,23-tetraamino substituted copper phthalocyanine is studied in 0.1 M Bu4NBF4and Bu4NClO4in dimethylformamide (DMF). The electropolymerization rate in DMF is two to three times that in dimethyl sulfoxide, due to a lesser donor ability of DMF and a weaker blocking of radical cations formed during the oxidation of initial copper phthalocyanine. According to a study of the electrochemical behavior of polymerized copper phthalocyanine in neutral aqueous solutions, the anodic doping of the polymer is accompanied by one redox transition. The two redox transitions, observed in an aqueous solution of acid, are explained by two successive steps of oxidation of an aniline-like structure or by the involvement of two protonated systems in the doping process, specifically, a stacked system (phthalocyanine ligands collected in parallel stacks) and a polyaniline system.

Electrostatic synthesis and electrochemical properties of a composition comprising ultrathin layers of silicododecamolybdate anions and poly(allyl ammonium) cations

A layer-by-layer (LbL) composition comprising ultrathin anionic layers of silicododecamolybdate and cationic layers of poly(allyl ammonium) is synthesized. The synthesis is realized by means of successive immersion of a glassy-carbon rod into aqueous sulfuric acid solutions of silicododecamolybdic acid and poly(allyl ammonium) hydrochloride. Cyclic voltammetry shows that the silicododecamolybdate anion in the composition undergoes three steps of reversible reduction with formal potentials of 0.34, 0.22, and 0.02 V (SCE). It is established that in the course of synthesis one can obtain a sixfold increase in the currents of redox conversions as compared with currents of a monolayer of the anion chemisorbed on glassy carbon. The LbL composition exhibits catalytic activity during electrochemical reduction of NO2−: the cathodic current of the third redox transition considerably increases and the peak in the reverse run of a cyclic voltammogram disappears. The calculated Michaelis constant of 5 × 10−2 M speaks of a high catalytic activity of the electrode.

Electrochemistry of fullerenes immobilized on the electrodes

The main methods of fullerene immobilization on solid electrodes are briefly reviewed. The effect of the procedures of immobilization and introduction of donor substituents into fullerene on the electrochemical (and spectral) characteristics is considered. A general classification of fullerene coatings is proposed. The coatings can be grouped as (i) the phase films of individual fullerenes and (ii) the composite films of fullerenes, which are dispersed in/on the matrixes of various natures. It is concluded that the main parameters of redox conversions of phase films are determined by the retardation of matrix restructurization in the doping and, in the case of dispersed fullerenes, by the character of their interaction with other components of composition. The correct results for the dispersed systems can be obtained only by using the reference electrode with a potential virtually independent of solvent nature.

Physicochemical features of poly- o -phenylenediamine modification with palladium microparticles

A new method is developed to prepare alladium dispersed in a polymeric matrix. Exposure of poly-o-phenylenediamine films in PdCl2 solution showed palladium cations strongly linked by the polymeric matrix to enable reduction of the metal accumulated in the polymer bulk. Optical absorption spectra were studied for poly-ophenylenediamine saturated with palladium ions and poly-o-phenylenediamine with palladium electroreduced upon impregnation. Method of X-ray photoelectron spectroscopy confirmed the presence of palladium cations in unreduced poly-o-phenylenediamine film and palladium metal upon reduction in polymeric matrix.