Electrohydrodynamic Impedance Research Articles

The theory of ac and EHD impedances for a mediated reaction on a redox polymer modified electrode

The ac and electrohydrodynamical (EHD) impedances have been calculated for an electrode modified by a redox polymer in contact with a solution containing a redox couple. The expressions obtained are functions of the diffusion coefficient of electrons in the film, the redox capacitance of the polymer film and its thickness, and the faradaic impedances on both sides of the film. These impedances, established for a rotating disc electrode by considering quantitatively convective diffusion effects, were simulated to illustrate the relative contribution of each process.

An ac and electrohydrodynamical (EHD) impedance investigation of redox processes occurring at polyaniline-coated electrodes

The redox processes of the [Fe(CN) 6] 3− 4− couple mediated by polyaniline films were studied by steady-state, ac and EHD impedance methods. At pH < 1, charging and discharging of the polymer capacitance in parallel with the faradaic impedance of the substrate reaction, occurring at the polymer-electrolyte interface, are the processes which determine the transient behaviour. Neither the charge transport across the film nor the charge-transfer kinetics of either the substrate or the polymer are experimentally accessible. At less acidic pHs (~ 3), a low frequency phenomenon detected in EHD experiments seems to be associated with surface protonation equilibria.

Investigation of tellurium electrocrystallization by EHD impedance technique

Electrohydrodynamical impedance measurements were performed during tellurium electrodeposition and correlated to the morphology of the deposits as observed by SEM. In particular, when the current is under a pure mass transport control, microdendrites are formed with various shapes depending on experimental conditions. EHD diagrams are then characteristic of a non-uniformly accessible surface. The calculated dimensions of active zones are in agreement with the sizes of the observed dendrite bushes.

Determination of the Diffusion Coefficient from Impedance Data in the Low Frequency Range

A new method is given for determining the diffusion coefficient or the Schmidt number of an electroactive species. The variation of the real part, relative to the imaginary part, of the electrochemical impedance, or of the electrohydrodynamic (EHD) impedance, at a rotating disk yields the diffusion coefficient without any assumption about the kinetics or any specific values for the electrode area or the solution concentration. This method may be used for all ordinary Schmidt number values (greater than 100).

Transient Mass Transfer at a Coated Rotating Disk Electrode: Diffusion and Electrohydrodynamical Impedances

The mass‐transfer response of a rotating disk electrode to a sine wave modulation of the angular velocity has been calculated for an interface covered by a porous layer of thickness , through which an ionic species is transported by molecular diffusion with a diffusivity . Electrohydrodynamical (EHD) impedances, defined as the transfer functions between the response of the concentration or concentration gradient at the interface and the modulated velocity, have been expressed as functions of a dimensionless frequency p, representing the ratio of the modulation frequency to the average angular velocity, of the permeability and of the diffusion time constant through the film . The frequency analysis of the impedance data for a coated electrode, therefore, allows one to determine and separately.

Electrochemical Behavior of Passive Iron in Acid Medium: II . The Passive Film/Solution Interface

The passive film on iron in acid media is investigated by means of a rapid change of electrolyte composition, with special attention to anion content, and by redox kinetics studies with a rotating disk under steady state and nonsteady state (electrohydrodynamic impedance). These techniques, associated with classical electrochemical impedance, allow one to establish the existence at the film‐electrolyte interface of a loosely structured film limiting the rate of mass transfer in the case of ferrocyanide oxidation. Dynamic exchange of anions and most of the electrolyte composition effects take place in this layer. An attempt is made to relate the behavior of the passive film on iron in acid media to basic properties of iron oxides and oxyhydroxides in their colloidal state.

The cathodic mass transport process during zinc corrosion in neutral aerated sodium sulphate solutions

The cathodic behaviour of zinc in 0.5 M Na 2So 4 aerated solution has been investigated using a rotating disc electrode. Since the steady-state current-voltage curves exhibit two well-separated diffusion plateaux, both steady-state (diffusion current vs. the disc angular velocity plts) and transient (frequency analysis of the electrohydrodynamical impedance) measurements which specifically sample mass-transport phenomena, have been carried out. It is shown that, in the vicinity of the corrosion potential (first plateau region) the dissolved oxygen reduction occures through a layer of dissolution products whereas the electrode surface appears quasi-uniformly active at the second plateau. Electrochemical impedance measurements corroborate the existence of the layer and allow us to justify the shape of the entire current-voltage curve. Finally, additional experiments have been carried out in order to gain a better understanding of the nature and the properties of the layer.

Corrosion Study of a Carbon Steel in Neutral Chloride Solutions by Impedance Techniques

The electrochemical behavior of a carbon steel in 3% solution has been investigated using a rotating disk electrode. Both steady‐state (diffusional current vs. the disk angular velocity plots) and transient (frequency analysis of the electrohydrodynamical impedance) measurements which specifically sample mass transport phenomena, have been carried out. It is shown that oxygen transport takes place not only in the liquid phase but also through a porous layer of corrosion products. From electrochemical impedance measurements, it was found that at the corrosion potential the oxygen reduction reaction is under either diffusional or mixed (activation + diffusion) control depending on both the electrode rotation speed and on the hold time at the free corrosion potential. In addition, it was shown that the oxygen consumption occurs not only by electrochemical reduction but also by chemical oxidation of ferrous to ferric ions. Finally, because of the possible occurrence of mixed corrosion control, it is emphasized that the use of the polarization resistance in order to evaluate corrosion rates is not always valid.

Electrohydrodynamical impedance on a rotating disk electrode Part II. Non-steady mass transfer in an ostwald fluid

The electrohydrodynamical impedance on a rotating disk defined as the ratio of the mass-transfer rate response to a small sine-wave modulation of the angular velocity has been calculated for an Ostwald fluid. This impedance, expressed as a function of the modulation frequency suitably reduced by means of the rheological parameters, is then identical to the expression derived for a newtonian fluid. An excellent agreement was found between these calculations and experimental data obtained for a viscous flow of a polyethylene oxide solution of high molecular weight.

Electrohydrodynamical impedance on a rotating disk electrode: Part II. Non-steady mass transfer in an Ostwald fluid

The electrohydrodynamical impedance on a rotating disk defined as the ratio of the mass-transfer rate response to a small sine-wave modulation of the angular velocity has been calculated for an Ostwald fluid. This impedance, expressed as a function of the modulation frequency suitably reduced by means of the rheological parameters, is then identical to the expression derived for a newtonian fluid. An excellent agreement was found between these calculations and experimental data obtained for a viscous flow of a polyethylene oxide solution of high molecular weight.

Electrohydrodynamical Impedance on a Rotating Disk Electrode: I . Redox System

Analytical expressions of the response of the observable electrical quantities (current or potential) to a sinusoidal perturbation of the rotation speed of a rotating disk electrode have been derived for a redox process. These expressions hold in the limiting diffusion current conditions as well as in the intermediate kinetics and whatever the regulating mode used (potentiostatic or galvanostatic). For a galvanostatic regulation the normalized electrohydrodynamical impedance only depends on the Schmidt number and does not depend on the electrochemical kinetics. On the other hand for a potentiostatic regulation the normalized electrohydrodynamical impedance depends both on the Schmidt number and on the electrochemical kinetics. Hence the expression of this last electrohydrodynamical impedance involves the usual electrochemical impedance. The analysis of the experimental results obtained by using two fast redox systems for high values of the Schmidt number and covering a wide frequency range shows a good agreement with the theoretical derivations.

Relationship between the electrochemical impedance and the electrohydrodynamical impedances measured using a rotating disc electrode

Relationship between the electrochemical impedance and the electrohydrodynamical impedances measured using a rotating disc electrode

Relationship between the electrochemical impedance and the electrohydrodynamical impedances measured using a rotating disc electrode

Relationship between the electrochemical impedance and the electrohydrodynamical impedances measured using a rotating disc electrode