I. Epelboin

Impedance measurements for nickel deposition in sulfate and chloride electrolytes

Abstract From the analysis of the impedance of nickel deposition, the electrode kinetics is shown to be dependent on the type of anion. In chloride electrolytes a slow electrode activation with cathodic polarization is predominant. In sulfate solutions a low-frequency capacitive feature, favored by a pH decrease, appears to result from interactions between the nickel and hydrogen discharges. An interpretation is proposed where the ad-ion Ni ads I acts as both a reaction intermediate and a catalyst associated with a propagating kink site, and where the adsorbed species H ads * , generated by the presence of Ni ads I , inhibit the hydrogen evolution. It is concluded that the active area is closely connected to the coverages by adsorbates.

Behavior of Secondary Lithium and Aluminum‐Lithium Electrodes in Propylene Carbonate

The improvement of the lithium cyclic efficiency obtained by substituting an aluminum substrate for a lithium substrate is explained by means of morphological and kinetic studies of the electrodes in the molar solution LiClO/sub 4/- propylene carbonate. SEM observations show that the insertion rate of the lithium deposit into aluminum can be sufficiently high to avoid the dendritic growth; ESCA analysis also reveals that the chemical decomposition of propylene carbonate to form a polymeric membrane is reduced on the electrode surface. Electrochemical impedance measurements associated with polarization curves data point out that most of the surface is active giving rise to an exchange current density of about 17 ma/cm/sup 2/. These studies also demonstrate that diffusion processes in the passivating layer and in the bulk of the electrode are responsible for the limited lithium cycling performances with the aluminum substrate. 18 refs.

Electrochemical noise generated by anodic dissolution or diffusion processes

Abstract A model of the noise generated by electrochemical reactions and by diffusion is proposed. The elementary fluctuations are supposed to be the particle fluxes which are Poisson white noise. This model is successfully used to describe the experimental stochastic behaviour of two cases of non-equilibrium electrochemical interfaces: the noise generated by anodic dissolution of iron in acidic medium and that by diffusion of a reacting species in the bulk of the electrolyte.

The Ag/Ag+ system: An impedance model for nucleation and growth

Abstract An analytical expression for the electrochemical impedance spectra is calculated, based on the assumption of successive monolayers generated at a mean nucleation rate and governed by the same ageing law. From simulated complex plane impedance plots, it is shown that this model can be applied to highly reversible metal-ion systems such as Ag + ⇋Ag.

The dissolution and passivation of Fe and Fe-Cr alloys in acidified sulphate medium: Influences of pH and Cr content

Abstract The steady state polarization curves of ferritic Fe Cr alloys, containing 7–12% Cr and immersed in 1M H 2 SO 4 , exhibit two current maxima, i.e. two passivating processes take place at the electrode interface within two different voltage ranges. The origin of these two passivation phenomena was investigated by plotting the steady state polarization curves and also by measuring the electrode impedance along with different polarization points. These results are then compared to those obtained with Fe electrode immersed in sulphate solutions of various pH values (0–5) since the steady state polarization curves of this electrode exhibit also two current maxima in a weakly acid medium, i.e. at solution pH greater than 4. However, even in a strongly acid solution in which only one current maximum is observed for the Fe electrode, the electrode impedance showed two passivation processes are occurring at the electrode interface. Therefore, the Fe dissolution involves at least two dissolution paths and the current at which one dissolution path overtakes the other is dependent upon the solution pH. The impedance diagrams of Fe Cr alloy electrode are similar to those of Fe electrode especially when the comparison is made with the Fe electrode immersed in a less acid solution, i.e. the addition of Cr in Fe enhances the appearance of two passivation phenomena with increase of solution pH.

Intermédiaires adsorbés et électrocristallisation

Abstract It is shown that the inductive impedance of an electrode where an electrocrystallization process takes place can be explained by the variation, with the potential, of the partial coverage of the electrode by adsorbed intermediates. On the other hand, this property of the electrochemical impedance cannot be related to the variation of the cathodic surface with the potential. The involvement of an adsorbed intermediate such as Zn ads + in an autocatalytic step can give rise to multiple steady-states. The morphology of spongy, compact or dendritic electrodeposits, which are observed successively when the current increases, is explained taking into account a coupling between the interfacial reactions and the surface diffusion of Zn ads + during the electrocrystallization.

Impédance électromécanique obtenue au courant limite de diffusion à partir d'une modulation sinusoidale de la vitesse de rotation d'une électrode à disque

Abstract We present an approach which permits the prediction of the limiting diffusion current of a polarographic reaction when the rotation speed of a disc electrode is sinusoidally modulated. For very low frequencies, we have calculated an expression for the electromechanical impedance making the assumption that the concentration profile is linear. After constructing high performance apparatus we have determined, in the case of a Newtonian medium, the numerical coefficient of a supplementary term extending the validity of the analytical expression of the electromechanical impedance to a wide range of frequencies. We have then been able to apply this method to the study of electrochemical kinetic problems in which the overall current is determined by convective diffusion. The case of the anodic behaviour of active iron in H2SO4 medium provides a good example. We determine then the diffusion component of a zero overall current (measured at the corrosion potential) and specify the value of the Schmidt number by the analysis of the electromechanical impedance as a function of frequency.

The Study of the Passivation Process by the Electrode Impedance Analysis

The passivity of metals has been known for a long time and an enormous amount of work has been done to characterize the nature of the passive state. An historical survey of the problem has been recently published(1) and modern contributions to the field can be found in the four International Symposia on Passivity(2–5) and in other meetings devoted to related topics.(6,7) In the course of the last decade, surface spectroscopy and optical techniques have improved our knowledge of the passive layer(8) During the same period, advanced electrochemical techniques become applicable to the solid electrode—electrolyte interface and were successfully introduced in the study of the anodic behavior of metals including the passivity phenomenon.(9) Among these techniques, the present chapter deals with ac impedance measurements,(10) extended to the subacoustic frequency range, which provide information about the kinetics of the passivation process.

Une nouvelle méthode potentiocinétique d'étude des processus électrochimiques rapides☆

Apparatus is described in which the essential element is an electronic potentiostat with a large band pass permitting the imposition of an electrode potential varying with time according to any desired programme. Current/potential (I = f(V) curves and the components Z and ϕ of the impedance Zc = Ze−iϕ of the cell can thus be observed or recorded either potentiostatically or potentiodynamically. The cell can also be given current or potential impulses. In particular, it is shown by a Fourier-series development that when a small square-wave ac potential is superimposed on dc polarization, the current—time variation determines the general shape of the complex impedance diagram as a function of frequency. Studying the transpassivity of chromium and of nickel in 2 N sulphuric acid at 26°C by way of example, we find in the case of rapid processes that this latest potentiodynamic method gives results in good agreement with those obtained by the measurement of the impedance Zc.

The use of rotating electrodes in the study of the different processes relevant in electrochemical engineering

Abstract The authors present recent results illustrating the developments in Electrochemical Engineering arising from the use of the rde system as devised by Levich since 1942. It is known, indeed, that when a fast electrochemical reaction is used, the control by mass transport from the bulk solution to the interface of the overall current, yields a way of studying hydrodynamics in the boundary layer thanks to the coupling due to convective diffusion. The examples presented below pertain in particular to boundary layer turbulence (measurement of eddy diffusivity and study of the local hydrodynamic drag reduction) as well as to convective diffusion in non Newtonian fluids with a prospect of application to Bioelectrochemistry. The authors underline also the improvements achieved in the methodology by describing recent non steady state methods.