Electrode geometry and Hopf instability

The method of impedance spectroscopy was used to study the effect of the electrode geometry and size on the appearance of spontaneous oscillations in the model electrochemical system of the N-NDR type, when the region of negative differential resistance is observed in the steady-state polarization curve. It is shown that in the case of the spherical and cylindrical electrodes, the potential region in which Hopf instability is observed becomes more narrow at a decrease in the electrode radius. However, this region will be wider in the latter case.     

Dynamic instabilities of model electrochemical system with electrocatalytic oxidation and preceding chemical reaction

The method of impedance spectroscopy was used to study the regularities of behavior of a model electrochemical system with an electrocatalytic reaction on a planar electrode which is preceded by a homogeneous chemical reaction. It is shown that in the case of low rates of the homogeneous chemical reaction, two types of bifurcations may exist in the system at the chosen attraction constant value; namely, the Hopf bifurcation leading to spontaneous oscillations and saddle-node bifurcation resulting in bistability of the system. The Hopf bifurcation disappears from the system at medium and high rates of the homogeneous chemical reaction, while the saddle-node bifurcation is retained.     

Adsorption from a Layer of Finite Thickness on a Planar Electrode

Within the Nernst diffusion model, the effect the convection has on the adsorption at a planar electrode is studied for the case of a diffusion-controlled stage of adsorption and the behavior of the Frumkin–Melik–Gaikazyan finite adsorption impedance is analyzed. Allowing for the convection leads to new functional frequency dependences of constituents of this impedance at low frequencies, where the active constituent depends on the diffusion layer thickness more heavily than the capacitive one. During adsorption of neutral molecules from a layer of finite thickness, an additional relaxation time emerges in an electrochemical system, which results from a finite rate of motion of species in the near-electrode layer. Ignoring the convection may lead to erroneous interpretation of the adsorption mechanism.     

Kinetics of Adsorption of Neutral Molecules from a Thin Layer on a Rough Electrode

A theoretical model for the generalized finite adsorption impedance of a rough electrode is offered. A complex function of the roughness of the electrode surface is introduced and its frequency dependence is studied. The frequency characteristics of constituents of the adsorption impedance are shown to depend on the electrode roughness and the diffusion layer thickness. The frequency dependences of the phase angle and impedance components are found to considerably differ from the Frumkin–Melik-Gaikazyan result for an ideal smooth electrode.     

Specific features of the behavior of an electrochemical system in the case of the Hopf instability for a spherical electrode

Model approximations are developed that allow establishing a quantitative relationship between the geometrical parameters of a spherical electrode, the faradaic impedance, and instabilities of the electrochemical system for an electrode reaction under potentiostatic conditions for the adsorption of species preceding their discharge. It is shown that the control parameter ω_H in the Hopf bifurcation point depends on the electrode size. The effect of the Nernst diffusion layer is observed at low frequencies in the range of negative faradaic impedance values.     

Effect of Adsorption of Neutral Molecules on the EDL Capacitance of a Rough Electrode

The perturbation theory is used to design a theoretical model for the Frumkin–Melik-Gaikazyan impedance of a sine electrode with a perturbation parameter equal to the ratio between the amplitude of surface oscillations and its period for a diffusion-controlled adsorption stage. The unequal diffusion access to the interface increases its capacitance. At higher frequencies, the electrode capacitance is defined by its geometric roughness factor. At lower frequencies, the capacitance depends on the adsorption conditions. The adsorption of electroactive substances results in a constant phase angle; the electrode roughness alters it and the frequency range where it remains constant.     

Transient Faraday process under a sinusoidal effect

Theoretical and Experimental Chemistry -     

On stability of model electrocatalytic process with frumkin adsorption isotherm occurring on spherical electrode

The method of impedance spectroscopy was used for theoretical studies of the conditions of appearance of Hopf instability in a model electrochemical system with a preceding homogeneous chemical reaction in the Nernst diffusion layer and electrocatalytic reaction on the spherical electrode surface under potentiostatic conditions. It is shown within the suggested electrochemical instability model based on the potential-dependent adsorption/desorption that the effective rate of the preceding homogeneous chemical reaction may affect the system stability. The effect diminishes at a decrease in the electrode radius. The instability region grows at an increase in the thickness of the Nernst diffusion layer.     

Effects of Biphenol and Quinone Derivatives on the Radiation-Initiated Dephosphorylation of 1-Glycerophosphate

The effects of biphenol and quinone derivatives on the radiation-initiated dephosphorylation of 1-glycerophosphate under various conditions were studied. It was found that in deaerated media, para-and ortho-quinone derivatives inhibited the formation of the phosphate ion by a factor of 1.8–1.9, and hydroquinone and pyrocatechol derivatives inhibited this reaction by a factor of 1.3–1.4, whereas 5-methylresorcinol and 2-methoxyphenol had no effect on the elimination of inorganic phosphate. It was demonstrated that a decrease in the radiation-chemical yields of the phosphate anion was due to the oxidation of 1-glycerophosphate radicals, which were formed under -radiolysis in aqueous solutions, by quinones.     

Determination of the effective concentration of free charge carriers in solid proton-conducting electrolytes

A method for the theoretical determination of the effective concentration of free charge carriers in solid proton-conducting electrolytes with the use of experimentally measured values of the electrical conductivity has been proposed. Polyantimonic acid has been considered as an example. It has been shown that the high electrical conductivity of solid proton-conducting electrolytes in comparison to other compounds with hydrogen bonds is due to the high concentration of effective free charge carriers and the low activation energy for the mobility of a proton.Translated from Teoreticheskaya i éxperimental'naya Khimiya, Vol. 24, No. 1, pp. 111–114, January–February, 1988.