György Inzelt

Electron and proton conducting polymers: recent developments and prospects

Abstract The most important topics of the rapidly developing field of conducting polymers are surveyed. Particular emphasis is laid on the problems of synthesis, structure, thermodynamics and kinetic behaviour of these systems. The relevant experiences, existing models and theories are outlined. Abundant examples of the growing applications are also discussed.

Polynuclear nickel hexacyanoferrates: monitoring of film growth and hydrated counter-cation flux/storage during redox reactions

Abstract An electrochemical quartz crystal microbalance (EQCM) was employed to monitor directly the growth of nickel(II) hexacyanoferrate(III) (NiHCNFe) films on gold substrates during electrodeposition as well as a result of sol-gel aggregation in colloidal nickel ferricyanide solutions used for modification. Frequency changes due to mass changes of the gold/crystal working electrode were correlated with cyclic voltammetric (CV) data. Evidence is also provided for the sorption of counter-cations (Li+, Na+ and K+), and associated water molecules, during redox reactions of the film. There is a strict relationship between the amount of alkali metal ions incorporated into the film during reduction, or excluded from the film during oxidation, and the frequency changes during EQCM measurements. The amount of solvent (H2O) transferred and sorbed in the NiHCNFe film reflects the degree of hydration of the investigated counter-ions. Anions also seem to participate in NiHCNFe electrochemistry, but their role is much less pronounced.

Electrochemical impedance spectroscopy of thin films with two mobile charge carriers: effects of the interfacial charging

In the electrochemical systems containing an excess of the background electrolyte, the faradaic process and the interfacial (‘double-layer’) charging are coupled to the fluxes of different charge carriers, the former being related to the diffusional transport of electroactive entities while the latter being realized mostly by ions of the supporting electrolyte. As a result, the interfacial capacitance Cdl may simply be added in parallel to the faradaic impedance specific for each particular system (Randles & Ershler). This simple treatment is not justified in the absence of an indifferent electrolyte, if the same charged species take part in both the electrode reaction and the double layer charging. This is the case where at least one of the ions of a binary electrolyte participates in the electron transfer process or cross the interface. A similar situation arises in the case of an electrochemically active polymer film in that mixed electronic–ionic conductivity prevails, i.e. the charging of the polymer (electron transport) is accompanied by the motion of the charge-compensating counterions (or co-ions). In such systems both interfacial processes are coupled with the same flux of the ‘electroactive’ component. Moreover, the distributions of both charged species inside the film are interrelated due to the electroneutrality condition and the self-consistent electric field so that their transport cannot be considered as pure diffusion. This paper presents an analysis of the alternating current passage across a thin film containing mobile charge carriers of two types, which for the sake of specificity are referred to as ‘electrons’ and ‘counterions’ (although the results are also valid for other systems with two charge carriers). The contributions at the interfaces related to the faradaic processes (or the ion exchange) and to the double-layer charging have been taken into consideration ‘ab initio’. Analytical expressions for the complex impedance have been obtained for three principal arrangements of the system, m ∣ f ∣ m′ (film between two electronic conductors), s′ ∣ f ∣ s (‘membrane’, film between two solutions) and m ∣ f ∣ s (modified electrode). Besides the parameters characterizing the transport processes of the charged species in the bulk film, these formulae contain four characteristics of each interface α (α=m ∣ f or f ∣ s), Rα (charge transfer resistance), Cα (interfacial capacitance), teα=1−tiα (‘double-layer numbers’) and Cμα (‘asymmetry factor’). The predicted complex-impedance plots demonstrate a greater variety of shapes compared to those predicted by traditional approaches.

Anion-involvement in electrochemical transformations of polyaniline. A radiotracer study

The sorption of labelled HSO−4 and SO2−4 ions into polyaniline films was studied by a radiotracer method. Simultaneous voltammetric and voltradiometric measurements and equilibrium sorption studies were carried out. It was found that the oxidation/reduction processes are accompanied by simultaneous changes in the anion sorption. Relationship between anion sorption and redox behaviour is discussed.

Role of polymeric properties in the electrochemical behaviour of redox polymer-modified electrodes

Abstract The influence of the polymer structure, and its changes during electrochemical transformation of redox sites in the film, on the electrochemical behaviour of polymer-modified electrodes, is discussed. Experimental results obtained by the variation of the site concentration, the nature and concentration of supporting electrolyte, solvent and temperature are reviewed. Theoretical and practical consequences are also emphasized.

Some problems connected with the study and evaluation of the effect of pH and electrolyte concentration on the behaviour of polyaniline film electrodes

Abstract The effect of the concentration of various supporting electrolytes (acid and salt) on the cyclic voltammetric behaviour of polyaniline film electrodes has been studied in aqueous solutions. The role of junction potential and the ionic activity in voltammetric response was analysed. Conditions for the reliable evaluation of experimental data are suggested.

Electrochemical quartz crystal microbalance study of the influence of the solution composition on the behaviour of poly(aniline) electrodes

Cyclic and potentiostatic electrochemical quartz crystal microbalance measurements are reported for poly(aniline) deposited on Pt electrodes and immersed in aqueous solutions of HClO4, 4-toluenesulfonic acid and 5-sulfosalicylic acid. The effect of solution composition on the mass changes occurring simultaneously with electrochemical transformations of the polymer was investigated. The results indicate that the pH and potential-dependent relative contribution of anions and cations to the overall ion exchange process is in connection with the degree of protonation of the reduced, partially and fully oxidised forms of the poly(aniline). The small mass change at lower potentials can be rationalised in terms of a model in which it is assumed that the expulsion of protons at the beginning of the first oxidation step and the incorporation of anions at a later stage of oxidation is basically determined by thermodynamics and not the kinetics of the system. Kinetic complications also arise for several reasons such as slow diffusion of ions and solvent molecules and relaxation of the polymer network. The actual film morphology depends on the nature of the solution used. It is proved that the pore size exclusion effect in the case of larger anions does not exist as thought previously, inasmuch as the originally compact structure formed in perchlorate solution can be transformed to a more open and swollen one under continuous cycling in the solution containing electrolyte of large ions.

An advanced model of the impedance of polymer film electrodes

Abstract A new model for the impedance of polymer film electrodes has been developed in that three mobile charge carriers are considered. It has been observed especially in the case of organic conductive polymer that beside the electron diffusion and counterion transport very often hydrogen ions also participate in the charge transport and charge transfer processes in the course of redox transformation of these polymers, i.e. a model describing this case is essential. It has been demonstrated that by the help of reasonable assumptions some of the mathematical problems can be overcome and an analytical solution can be obtained for the complex impedance in the case of low amplitude variation of the electrode polarization. The equations derived for the charge transport and charge transfer impedances are tested by CNLS fitting data obtained for an Au∣poly( o -phenylenediamine) electrode.

Studies of the formation and redox transformation of poly(o-phenylenediamine) films using a quartz crystal microbalance

The electropolymerization of o-phenylenediamine on gold and the redox behaviour of poly(o-phenylenediamine) films were studied in acid media using the electrochemical quartz crystal microbalance technique. It was established that the film formation is not an autocatalytic process and takes place mainly during the negative-going potential scan. The results indicate that the films are swollen by solvent molecules and ions in both reduced and oxidized states, and the extent of swelling increases with film thickness. At least two electron transfer steps should be considered in the redox mechanism. In the first oxidation step anion insertion and/or proton expulsion occur, while in the second oxidation step anions and/or protons are expelled. The magnitude of the mass changes accompanying the redox transformations depends on the pH of the bulk solution, which indicates that protonation—deprotonation processes play an important role in the overall mechanism. Dimerization of sites of different oxidation states may also occur. A counter-flux of water molecules can be assumed during the second oxidation step, i.e. the fully oxidized films are more hydrated.

Handbook of reference electrodes

Electrode potentials.- Reference redox systems in non-aqueous systems and the relation of electrode potentials in non-aqueous and mixed solvents to standard potentials in water.- Liquid junction potentials.- Salt bridges and diaphragms.- Reference electrodes for aqueous solutions.- Reference Electrodes for Use in Nonaqueous Solutions.- Reference electrodes for ionic liquids and molten salts.- Reference Electrodes in Oxidic Glass Melts.- Reference electrodes for solid electrolyte devices.- Direct solid contact in reference electrodes.- Micro reference electrodes.- Conducting polymer based reference electrodes.- Screen-printed, disposable, reference electrodes.- Pseudo-reference electrodes.- The Kelvin Probe technique as reference electrode for application on thin and ultra-thin electrolyte films.