J. Navarro-Laboulais

Electrochemical behaviour of poly(neutral red) on an ITO electrode

Abstract The formation of a radical cation, by electrochemical oxidation of the neutral red dye in acidic aqueous solution, is controlled by diffusion towards the electrode surface. This is the initiation step of the electrogeneration, potentiostatic or potentiodynamic, of poly(neutral red) films on an indium–tin oxide (ITO) electrode. The polymeric film is a redox semi-conductor, and shows a microporous membrane-like character. Its electrochemical behaviour depends on the generation process. The hydrogen ions play a double role: first as reactants at electrochemical and acid–base equilibrium, second as counterions of the electrons. The global process of the electron-hopping is limited by the low electronic conductivity of the material. In spite of this, the thin films deposited on the ITO electrode can act as a pH potentiometric sensor.

Electrochemical study of Nafion membranes/Prussian blue films on ITO electrodes

Nafion (perfluorinated ion-exchange membrane) films allow the transport of counterions from the solution to the PB film necessary during the reduction and oxidation processes to maintain the film electroneutrality. Nafion films cause a decrease in the apparent diffusion coefficient. This apparent diffusion coefficient could be interpreted as a measurement of counterions transport through films. Prussian blue films stability is increased by the presence of Nafion films allowing the study of the electrochemical behaviour of the oxidized form of Prussian blue films (Berlin green, Prussian yellow) by impedance spectroscopy.

Charge transport in prussian blue films deposited on ito electrodes

Abstract Charge transport processes that can occur in Prussian Blue films deposited on ITO (indiumtin oxide) electrodes in contact with aqueous solutions have been studied. The concentration in the solution of some ions such as the proton or the potassium influence the films conductivity as well as the degree of hydration of the film. This points to the fact that Prussian Blue films act as ionic conductors when they are immersed in aqueous solutions. Furthermore, the apparent diffusion coefficient calculated from impedance spectra of PB films increases with the temperature which confirms the idea of an ionic conductor. The impedance spectra for the Everitts Salt films, however, differ from the PB films and is much less sensitive to temperature.

The correlation between electrochemical impedance spectra and voltammograms of PB films in aqueous NH4Cl and CsCl

Abstract Voltammetric curves of reduction of Prussian Blue Films to the Everitt’s Salt form and oxidation to the Prussian Yellow form show important differences depending on the supporting electrolyte countercation. The rate of the overall electron-hopping depends on the countercation. Apparent diffusion coefficients are evaluated from impedance spectra at different stabilization potentials. The dependence of the apparent diffusion coefficients on the stabilization potential can help understand the dependence of the voltammograms shape on the countercation nature.

Chronoamperometry of prussian blue films on ITO electrodes: ohmic drop and film thickness effect

Abstract The chronoamperograms associated with the reduction of prussian blue films deposited onto indium tin oxide (ITO) electrodes to the Everitt’s salt form, are influenced by the ohmic drop effect. These chronoamperometric curves have been simulated by means of a numerical finite difference model which is able to explain their shape and their dependence on the thickness of the film and on the uncompensated resistance. An analytical expression which describes the dependence of current against time at initial times considering the ohmic drop effect has also been proved when applied to these chronoamperometric curves at short times.

Electrochemical impedance spectroscopy of conductor—insulator composite electrodes: Properties in the blocking and diffusive regimes

Abstract The electrochemical response of graphite + high-density polyethylene composite electrodes as a function of the conductivity load was investigated. Percolation theory was used in order to explain the electrochemical behaviour of this type of composite electrode. In the blocking regime the electrochemical impedance of this electrode material behaved as R 0 + q · ( ω j) − η , where R 0 represents the uncompensated resistance of the cell. Its value depended on the graphite volume proportion ( ν ) with a power law R 0 ∞ ( ν — ν c ) − t with a critical exponent t = 3.2 ± 0.1 which is close to the mean field value, t = 3. With potassium chloride concentrations greater than 0.7 M, the uncompensated resistance, R 0 , coincided practically with the bulk resistivity of the solid. The electrochemical response of potassium ferricyanide was analyzed using a generalized Randles equivalent circuit where the double layer capacity and the Warburg impedance were substituted by two independent CPEs, respectively. In this diffusive regime and in the blocking regime, the values of the constants and exponents of the constant phase elements were interpreted on the basis of a surface model of composite electrodes and its disorder.

A numerical approach to the voltammograms of the reduction of Prussian Blue films on ITO electrodes

The uncompensated resistance, mainly due to the ITO electrode, modifies the shape of voltammetric curves of the system Prussian Blue ⇄ Everitts Salt films deposited on this transparent electrode. A numerical finite difference model which is able to explain the shape of these voltammetric curves is studied in this paper. This model explains the dependence of voltammetric curves on the film thickness and uncompensated resistance.

The role of potassium and hydrogen ions in the Prussian Blue ⇄ Everitt's Salt process.

Cyclic voltammetry experiments have proved very useful in the knowledge and understanding of the role of the potassium and hydrogen ions during the Prussian Blue ⇄ Everitts Salt process. While potassium ions act as a stoichiometric reactant, the hydrogen ions play a very important role in the kinetics of the electron-hopping process. The entropy change for the reaction PB ⇄ ES has proved to be dependent on the degree of crystallinity of the film, indicating that this entropy change is related to the fixation of inner counterions on the crystal structure of the films.

Ohmic drop effect on the voltammetric behaviour of graphite + polyethylene composite electrodes

Graphite + polyethylene composites can be used as electrodes in electrochemical experiments because of their low charging current in contrast with other composite electrodes. However, the results reveal that the ohmic drop must always be considered in the data analysis. The peak potential and the peak current are related through an expression deduced for a Nernstian process where the mass transport is controlled by diffusion. The determination of the uncompensated resistance implies also the calculation of the convolution of the current.

Voltammetric determination of optimal conductive load proportion in graphite—epoxy composite electrodes

Abstract The electrochemical behaviour of graphite—epoxy composite electrodes with various load ratios has been investigated by means of cyclic voltammetry. The optimal proportions of the conductive load in these materials have been worked out on the basis of calculations of double-layer capacity and non-faradaic electric charge values. An optimal graphite: epoxy ratio of 60 wt.% has been obtained and electroanalytical application of the composite has been studied in ferrocyanide standard solution.