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An atypical example of the effect of the self-induced convection on the morphology of current-potential responses was described, for the case of the parallel electroreduction of some metal cations (Cd 2+ , Cu 2+ , Zn 2+ and Eu 3+ ) and H + ions at the mercury electrode. Separation of the individual faradaic contributions from the metal ions and hydrogen ions reduction, made with the double potential-step chronocoulometry, and based on the difference between the rates of these processes, allowed us to find that the total faradaic current exhibits negative or positive deviations from the sum of individual faradaic currents. In order to diagnose the origin of this anomaly, the series of experiments for various acid concentrations, electrolysis times and the electrode types/geometries, including the hanging mercury drop and the mercury film electrodes, were performed. The results obtained indicated that the observed anomalies are due not to the surface blocking or latent limiting currents, but due to a rather novel mechanism that involves convective instabilities which accompany the hydrogen evolution reaction. The resulting change of the concentration distribution of all species involved in the electrode process appropriately changes the individual contributions from the faradaic currents. These studies show one more role of convection in electrochemical systems.

Electrochemical and electrochemical quartz crystal microbalance (EQCM) methods were used for the analysis of the growth and dissolution processes of sparingly soluble deposits. As a model of a sparingly soluble film, uranyl-hexacyanoferrate was selected. Additionally the rate of Prussian blue deposition was studied. We have shown that coupling of the EQCM technique with standard electrochemical methods may be useful in the study of mechanisms of electrodeposition phenomena when the solubility of the product of the electrode reaction is limited. The transformation of mass–time data into current–time dependences allows an analysis of the reaction in terms of electrocrystallization steps while other masking processes are superimposed during the polarization of the electrode. This approach may have general value and may be applied not only to hexacyanoferrate deposition but also to deposition of other products of the electrode reaction, which exhibit limited solubility. Analysis of growth of uranyl-hexacyanoferrate on the Au/quartz electrode under potentiostatic and cyclic voltammetry conditions, taken as an instance, reveals that the first step, limited by the growth of the crystallization nuclei, is clearly separated from the uniform growth, limited by diffusion. For thicker films, the slope of the frequency–charge dependence is dominated by the insertion/removal process of potassium ions, to and from the film. During the dissolution of the uranyl-hexacyanoferrate deposits, two clearly separated steps were observed. During the first, relatively fast oxidation of the deposit is observed which is accompanied by removal of potassium ions from the film. The second step corresponds to a chemical dissolution of the oxidized film. The rate coefficients corresponding to both steps were determined. Based on the gravimetric and electrochemical data, the molar mass of the deposited uranyl-hexacyanoferrate was estimated as equal to 690 ± 90 g/mol. From these and the EDX data, a chemical formula of the deposited uranyl-hexacyanoferrate was proposed as: K2(UO2)[Fe(CN)6].

The electrochemical behaviour of mixtures of phenazine (P) with iodine (I2) in the solid state was investigated. Three types of electrochemical experiments were conducted: (i) EMF measurements of the Ag/AgI/I2:P system for various I2 : P ratios, (ii) cyclic voltammetric (CV) experiments and (iii) potentiometric measurements of the open cell potentials after different cathodic charges were injected into the I2 : P pellets. In the first case, a detailed analysis of the observed electromotive force was presented. The Gibbs free energy of the I2 : P complex determined was in good agreement with earlier results. In the second case, the CV curves corresponding to the electrode processes of phenazine mixed with graphite in the solid state indicate that the mass and charge transfer, which occur in the pellets, are faster than expected. The potentiometric experiments confirm this conclusion. Additional experiments, conducted with pellets of different thickness, indicate that although, in comparison with an equilibrated pellet, the concentration profiles are modified in small layers (diffusion layers) neighbouring the electrodes, the electrode reaction probably occurs in the whole pellet. A tentative mechanism of these processes is discussed. Copyright © 2003 John Wiley & Sons, Ltd.

The temperature dependences of the four transfer coefficients for cathodic and anodic electrode reactions of two different types of systems, namely Cr(III)/Cr(II) and Cr(II)/Cr(Hg), were determined in 5.0 M NaClO 4 solution. The results were obtained for a wide range of temperatures from −20 to 90°C. The α parameter determined from the log k fh on E dependence corresponding to the Cr 3+ +e − →Cr 2+ electrode process shows the highest dependence on temperature (∂ α /∂ T =1.1±0.3×10 −3 K −1 ), whereas the β parameter corresponding to the Cr 2+ →Cr 3+ +e − electrode process is virtually independent of temperature (∂ β /∂ T =0). The same results were obtained when the transfer coefficients were determined from dependence of the enthalpy and entropy of activation on overpotential. The observed temperature dependence of α is attributed to the changes of the ϕ 2 potential. The transfer coefficients for the Cr 2+/0 electrode reaction also show some dependence on temperature (∂ α /∂ T =8.7±2.0×10 −4 K −1 and ∂ β /∂ T =3.2±1.4×10 −4 K −1 ), which probably results from the influence of the ϕ 2 potential.

Impedance spectroscopy was used to measure the properties of the gold electrode in 0.5 M H 2 SO 4 solution in the double-layer region (−0.25 to 1.05 V vs. sodium chloride saturated calomel electrode (SSCE)) and the region of oxide monolayer formation (1.05–1.4 V vs. SSCE). The dependence of the capacitance on potential was analysed. In both regions equivalent circuits composed of a resistor corresponding to the resistivity of the solution and a capacitor were proposed. It was found that in the double-layer region the capacitance reflected the properties of the pure gold electrode, whereas in the oxide region it depended on the coverage of the surface, corresponding partially to pure gold, MOH dipoles, oxide or a reconstructed gold surface. The improvements made by introducing a constant phase element instead of the capacitor and adding a parallel resistor in the oxide region were analysed. The roughness of the gold electrode was also estimated. The results obtained by impedance measurements were complemented by voltammetric studies to clarify the process, to confirm the measured values and to explain the meanings of the calculated parameters.

Graphite electrodes intercalated with various metals (Ni, Co, Pd, Pt, Rh, and Ru) have been bonded with an inorganic polymer and used as cathodes in the hydrogen evolution reactions (HER) in . Four of the most active electrodes showed very good mechanical and electrochemical stability. The overvoltage of the HER at decreased in the following order: −525 mV, for pure graphite and −254, −137, −103, and −58 mV, for the Pd/C, Rh/C, Pt/C, and Ru/C electrodes, respectively. The kinetics of the HER for two electrodes (Ru/C and Pd/C) were measured with the use of an ac impedance technique. In both cases, the HER proceeds via the Volmer‐Heyrovsky mechanism. The results obtained proved that the Pd/C electrode had a larger active surface area than the Ru/C one did.

Graphite electrodes intercalated with various metals (Ni, Co, Pd, Pt, Rh, and Ru) have been bonded with an inorganic polymer and used as cathodes in the hydrogen evolution reactions (HER) in . Four of the most active electrodes showed very good mechanical and electrochemical stability. The overvoltage of the HER at decreased in the following order: −525 mV, for pure graphite and −254, −137, −103, and −58 mV, for the Pd/C, Rh/C, Pt/C, and Ru/C electrodes, respectively. The kinetics of the HER for two electrodes (Ru/C and Pd/C) were measured with the use of an ac impedance technique. In both cases, the HER proceeds via the Volmer‐Heyrovsky mechanism. The results obtained proved that the Pd/C electrode had a larger active surface area than the Ru/C one did.

Donnees sur le processus d'electrode Cr(II)+2e − =Cr(O) en presence d'une electrode de mercure, souvent observe comme 2eme etape de la reduction d'ions Cr(III), entre pH 2 et 4

The electrode process Bi(III)+ 3e = Bi(Hg) has been investigated in weakly acidic, neutral and alkaline solutions by means of amalgam polarography and cyclic voltammetry and chronopotentiometry with the use of hanging amalgam drop electrode. It has been found that in weakly acidic and neutral solutions, oxidation of bismuth amalgam goes through aquocomplexes of bismuth. In the cathodic step on cyclic voltammetry curves four peaks of reduction are observed, with magnitude and shape strongly dependent on pH of the solution. These peaks have been ascribed to the reduction of Bi3+, Bi(OH)+2, Bi(OH)+2 and Bi(OH)3. In alkaline solutions, oxidation of bismuth amalgam leads to Bi(OH)3. In cathodic scan on cyclic voltammetry curves only one peak is observed, which corresponds to Bi(OH)3 reduction. Formal potentials of electrode process and solubility product of Bi(OH)3 have been calculated and discussed. The step-wise nature of the overall electrode reaction has been ascertained and discussed. At higher concentrations of OH− ions, Bi(OH)3 dissolves.

The electrode process Bi(III)+ 3e = Bi(Hg) has been investigated in weakly acidic, neutral and alkaline solutions by means of amalgam polarography and cyclic voltammetry and chronopotentiometry with the use of hanging amalgam drop electrode. It has been found that in weakly acidic and neutral solutions, oxidation of bismuth amalgam goes through aquocomplexes of bismuth. In the cathodic step on cyclic voltammetry curves four peaks of reduction are observed, with magnitude and shape strongly dependent on pH of the solution. These peaks have been ascribed to the reduction of Bi3+, Bi(OH)+2, Bi(OH)+2 and Bi(OH)3. In alkaline solutions, oxidation of bismuth amalgam leads to Bi(OH)3. In cathodic scan on cyclic voltammetry curves only one peak is observed, which corresponds to Bi(OH)3 reduction. Formal potentials of electrode process and solubility product of Bi(OH)3 have been calculated and discussed. The step-wise nature of the overall electrode reaction has been ascertained and discussed. At higher concentrations of OH− ions, Bi(OH)3 dissolves.