Vincent Vivier

Constant-Phase-Element Behavior Caused by Resistivity Distributions in Films I. Theory

I. Theory Bryan Hirschorn,* Mark E. Orazem,** Bernard Tribollet,** Vincent Vivier,*** Isabelle Frateur, and Marco Musiani*** Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611, USA Laboratoire Interfaces et Systemes Electrochimiques, UPR 15 du CNRS, Université Pierre et Marie Curie, 75252 Paris cedex 05, France Laboratoire de Physico-Chimie des Surfaces, UMR CNRS-ENSCP 7045, Ecole Nationale Supérieure de Chimie de Paris, Chimie ParisTech, 75005 Paris, France Istituto per l’Energetica e le Interfasi, Consiglio Nazionale delle Ricerche, 35127 Padova, Italy

An Impedance Investigation of the Mechanism of Pure Magnesium Corrosion in Sodium Sulfate Solutions

The corrosion behavior of pure magnesium in sodium sulfate solutions was investigated using voltammetry and electrochemical impedance spectroscopy with a rotating disk electrode. The analysis of impedance data obtained at the corrosion potential was consistent with the hypothesis that Mg corrosion is controlled by the presence of a very thin oxide film, probably MgO, and that the dissolution occurs at film-free spots only. This hypothesis was substantiated both by the superposition of the EIS diagrams, obtained for different immersion times and for two Na2SO4 concentrations once normalized, and by use of scanning electrochemical microscopy in the ac mode to sense the local conductivity of the material. On the basis of the electrochemical results, a model was proposed to describe magnesium corrosion at the open-circuit potential. Simulation of the impedance diagrams was in good agreement with the experimental results.

Constant-Phase-Element Behavior Caused by Resistivity Distributions in Films II. Applications

II. Applications Bryan Hirschorn,* Mark E. Orazem,** Bernard Tribollet,** Vincent Vivier,*** Isabelle Frateur, and Marco Musiani*** Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611, USA Laboratoire Interfaces et Systemes Electrochimiques, UPR 15 du CNRS, Université Pierre et Marie Curie, 75252 Paris cedex 05, France Laboratoire de Physico-Chimie des Surfaces, UMR CNRS-ENSCP 7045, Ecole Nationale Supérieure de Chimie de Paris, Chimie ParisTech, 75005 Paris, France Istituto per l’Energetica e le Interfasi, Consiglio Nazionale delle Ricerche, 35127 Padova, Italy

The Apparent Constant-Phase-Element Behavior of a Disk Electrode with Faradaic Reactions

Geometry-induced current and potential distributions modify the global impedance response of a disk electrode subject to faradaic reactions. The problem was treated for both linear and Tafel kinetic regimes. The apparent capacity of a disk electrode embedded in an insulating plane was shown to vary considerably with frequency. At frequencies above the characteristic frequency for the faradaic reaction, the global impedance response has a quasi-constant-phase element (CPE) character, but with a CPE coefficient alpha that is a function of both dimensionless frequency K and dimensionless current density J. For small values of J, alpha approached unity, whereas, for larger values of J, alpha reached values near 0.78. The calculated values of alpha are typical of those obtained in impedance measurements on disk electrodes. For determining the interfacial capacitance, the influence of current and potential distributions on the impedance response cannot be neglected, even if the apparent CPE exponent alpha has values close to unity. Several methods taken from the literature were tested to determine their suitability for extracting interfacial capacitance values from impedance data on disk electrodes. The best results were obtained using a formula which accounted for both ohmic and charge-transfer resistances.

The Apparent Constant-Phase-Element Behavior of an Ideally Polarized Blocking Electrode

Two numerical methods were used to calculate the influence of geometry-induced current and potential distributions on the impedance response of an ideally polarized disk electrode. A coherent notation is proposed for local and global impedance which accounts for global, local, local interfacial, and both global and local ohmic impedances. The local and ohmic impedances are shown to provide insight into the frequency dispersion associated with the geometry of disk electrodes. The high-frequency global impedance response has the appearance of a constant-phase element CPE but can be considered to be only an apparent CPE because the CPE exponent is a function of frequency.

The Global and Local Impedance Response of a Blocking Disk Electrode with Local Constant-Phase-Element Behavior

Numerical methods were used to calculate the influence of geometry-induced current and potential distributions on the impedance response of a blocking disk electrode with a local constant-phase element behavior. While the calculated global impedance is purely capacitive, the local impedance has high-frequency inductive loops that were observed in experiments conducted on a stainless steel electrode in 0.05 M NaCI + 0.005 M Na 2 SO 4 electrolyte. The calculated global impedance responses are in good agreement with experimental results obtained using both the steel electrode and a glassy-carbon disk in KCl electrolytes of differing concentrations. The computed local and both local and global ohmic impedances are shown to provide insight into the frequency dispersion associated with the geometry of disk electrodes.

EIS investigation of zinc dissolution in aerated sulfate medium. Part I: bulk zinc

Abstract The kinetics of anodic dissolution and corrosion of zinc are investigated by electrochemical impedance spectroscopy in sulfate medium for various surface preparation conditions and different experimental procedures. A new version of the reaction model, which was established previously, accounts well for the experimental results. It implies several parallel paths of dissolution and three adsorbed intermediates: Zn+ad, Zn2+ad and ZnOHad. From the simulation of impedance data are deduced the values of quantities such as the partial current intensities of each parallel paths or the surface coverages by three adsorbates. It is shown that the surface preparation conditions may affect the balance between the competitive dissolution paths. When zinc is immersed in the electrolyte, a protective layer is formed, which inhibits the direct dissolution of the metal, but which tends to be eliminated by a potential activation. These phenomena are time dependent and may be affected by the slow formation of a corrosion product layer.

Electrochemistry of powder material studied by means of the cavity microelectrode (CME)

The kinetic aspects of powder material electrochemistry can be studied using the cavity microelectrode (CME) as it allows carrying out voltammetry at scan rates between a few millivolts per second to several hundreds of volts per second. Thus, significant voltammogram characteristics-scan rate profiles can be drawn. Theoretical models suited to each material needs to be developed for their exploitation. First, we report significant results obtained with CME on powder materials. The materials studied were chosen for their wide variety of possible applications such as battery materials (polyaniline or Bi2O3, which modifies the electrochemical behavior of materials in which it is included), supercapacitor (carbon black), and for the electrocatalytic hydrogenation of organic compounds (PtO2). Secondly, we briefly describe the general action for establishing models to obtain a better understanding of the electrochemical processes.

Cyclic voltammetry study of bismuth oxide Bi2O3 powder by means of a cavity microelectrode coupled with Raman microspectrometry

An electrochemical study of the bismuth oxide Bi2O3 with a cavity microelectrode coupled in situ to Raman microspectroscopy allowed species transformations during the redox behavior of this oxide to be identified. It was shown that the electroactive species are probably bismuth oxy-hydroxides rather than Bi2O3 when a potential is applied, whereas they are Bi2O3 on open circuit voltage.

EIS investigation of zinc dissolution in aerated sulphate medium. Part II: zinc coatings

The kinetics of anodic dissolution and corrosion of zinc coatings deposited onto steel sheet either by electrodeposition or by hot dipping are investigated by electrochemical impedance spectroscopy in aerated sulfate medium. The results are compared with those obtained previously on pure bulk zinc and interpreted on the basis of the model derived in the first part of this paper. It is shown that zinc coatings are less sensitive to corrosion than pure bulk zinc and changes of their behavior with time are not identical. Important differences are observed between the various coatings in the respective contributions of the three parallel paths of the dissolution process. Three kinds of oxidation products were identified by Raman spectroscopy. A compact non-stoichiometric zinc oxide was formed by surface reaction on zinc. Above it, a thick and porous layer made of zinc hydroxi-sulfate and stoichiometric ZnO, was formed by precipitation from a local saturation of the solution. A strong correlation was evidenced between the oxidation products and the various paths of the reaction model. It was assumed that the impurities, initially present in the metal, may affect the interfacial reactions, the increase of the micro-roughness, and may also reinforce the protective properties of oxidation product layers. The differences between the various zinc coating behaviors result mainly from their impurities. Their crystal preferred orientations have no significant influence.