Győző G. Láng

Some interesting aspects of the catalytic and electrocatalytic reduction of perchlorate ions

The problem of the reduction of perchlorate ions is discussed in the light of recent results bearing in mind the perchlorate environmental contamination challenge and recent problems concerning the electrochemical stability of ClO4− ions. The catalytic and electrocatalytic transformations are compared in the case of WC and Pt electrodes (catalysts). Various methods for the detection of the occurrence of the reduction process are presented (radiotracer technique, potential step method and voltammetry). It is shown that the interpretation of the results of studies of corrosion processes of Al, Zn, Cu, Ni and Fe carried out in the presence of ClO4− ions cannot be considered as adequate without taking into consideration the reduction of ClO4− ions and all previous work should be revisited on the basis of the new findings. Some elements of the mechanistic aspects of the complicated reduction processes are discussed.

Model dependence and reliability of the electrochemical quantities derived from the measured impedance spectra of polymer modified electrodes

Abstract Electrochemical impedance spectra obtained for poly(vinylferrocene) films are analyzed on the basis of selected theoretical models for the impedance of polymer-modified electrodes. The Kramers-Kronig transformation has been used for validation of the impedance data. Complex non-linear least squares fitting and computer simulation have been used to derive reliable quantities characterizing the electrochemical behaviour of these systems. It is demonstrated that the goodness of fit should be carefully checked in order to estimate parameters with adequate precision. It is emphasized that a relatively good fit concerning an impedance spectrum is a necessary, but not sufficient condition for the suitability of a model. Additional experimental test is required, e.g. measurements at a range of potentials, temperatures and film thicknesses.

New EQCM, voltammetric and radiotracer evidences proving the role of Cu+ ions in the behavior of the Cu2+–Cu system

Abstract Combined EQCM and voltammetric studies in conjunction with radiotracer adsorption investigations were carried out in order to obtain direct information on the interfacial behavior of the Cu 2+ –Cu system under equilibrium and dynamic conditions with the aim of clarifying the role and participation of Cu + ions formed by the interaction of cupric ions and copper metal. The appearance and accumulation of Cu + ions in the solution phase is clearly demonstrated through radiotracer experiments using labeled Cl − ions. The formation of bulk CuCl phase is preceded by the formation of a CuCl surface layer. Data obtained from combined voltammetric and EQCM studies allow us to separate the potential regimes where the electrosorption, the formation of Cu + or Cu 2+ ions, and dissolution of CuCl are the predominant processes.

Can the internal energy function of solid interfaces be of a non-homogeneous nature?

Abstract Equilibrium thermodynamics rests on the assumption that fundamental equations exist containing all thermodynamic information about a given system. Fundamental relations describing the thermodynamic properties of homogeneous phases are homogeneous functions of the first degree in the extensive variables. Homogeneous functions obey Euler’s theorem. This is also true for the surface thermodynamics of stressed solids. The derivation of a ‘generalised Lippmann equation’ for stressed solids first proposed by Couchman et al. and adopted by IUPAC violates the mathematical requirements for a thermodynamic equation and thus cannot be valid.

Effect of the temperature on the conductivity and capacitance of poly(aniline) film electrodes

Abstract The temperature and potential dependences of the behaviour of poly(aniline) modified electrodes were studied by impedance spectroscopy. It was rendered probable that the ohmic resistance which can be determined by impedance measurements is in connection with ionic conductivity and practically no information of the electronic conductivity of the polymer can be obtained. No temperature dependence of the low-frequency limiting capacitance was found. The high-frequency capacitance which can be assigned to double-layer at the polymer/electrolyte interface depends strongly on temperature. Both capacitances show cpe behaviour.

Remarks on the energetics of interfaces exhibiting constant phase element behaviour

Abstract The energetics of electrified interfaces exhibiting constant phase element (CPE) behaviour is discussed. In the double layer region energy can be dissipated in various ways other than by charge transfer. Therefore, contrary to the statement in Zoltowski, J. Electroanal. Chem. 443 (1998) 149, an electrode with a double layer of CPE character can be an ideally polarisable electrode.

Impedance analysis of poly(o-phenylenediamine) electrodes

Electrochemical impedance spectra obtained for poly(o-phenylenediamine) films in contact with solutions of different compositions were analyzed on the basis of selected models. The Kramers-Kronig transformation has been used for validation of the impedance data. Complex non-linear least squares fitting and computer simulation have been used to derive reliable quantities characterising the electrochemical behaviour of these systems. It has been shown that the charge transport diffusion coefficient strongly decreases with increasing potentials. The potential dependence of low-frequency capacitance attests to strong interactions within the film that depends upon the nature of anions. The participation of protons in the redox reactions has also been demonstrated.

On the reduction of ClO4− ions in the course of metal dissolution in HClO4 solutions

The problem of the application of perchloric acid and/or perchlorate supporting electrolyte in electrochemical and corrosion studies is discussed. It is demonstrated that in contrast to the general view in the literature the reduction of perchlorate ions could be an important step in the overall process occurring in the course of spontaneous dissolution of some metals in acid perchlorate medium. It is shown that especially in the case of the dissolution of iron the reduction of ClO4− leading to the formation of Cl− plays a substantial role. Consequently, all interpretations concerning the dissolution of iron in the presence of ClO4− ions should be revisited taking into consideration the possibility of the reduction process.

The problem of the low-frequency dispersion in the case of polymer film electrodes — an experimental impedance study on Au|poly(o-phenylenediamine) electrodes

Abstract Electrochemical impedance spectra are presented for Au|poly( o -phenylenediamine) electrodes in contact with 1 mol dm −3 perchloric acid solution. The roughness of the gold substrate and the film thickness, respectively, were systematically varied. The results obtained affirm that the dispersion of the low-frequency capacitance is connected with the inhomogeneity of the surface film, and the role of the surface roughness of the gold substrate is minor, if any, in the development of the CPE behaviour, which is in contrast to some hypotheses presented recently.

Application of the Kramers-Kronig transformation for the data validation of impedance spectra of electroactive polymer films on electrodes

Abstract A procedure concerning the application of Kramers-Kronig transformation for the validation of the impedance data in the case of thin electroactive films on electrodes is described. The treatment is based on a hypothetical shunting of the network corresponding to the polymer film electrode and the computer simulation of the altered impedance spectra.