CPE analysis by local electrochemical impedance spectroscopy

Constant-phase elements (CPE) are used extensively in equivalent electrical circuits for fitting of experimental impedance data. The CPE behavior is generally attributed to distributed surface reactivity, surface inhomogeneity, roughness or fractal geometry, electrode porosity, and to current and potential distributions associated with electrode geometry. In this work, different electrochemical systems showing the CPE dependence in the high-frequency range for the overall impedance were considered. Local electrochemical impedance spectroscopy was found to provide a good means for assessing the influence of local variations on the CPE behavior seen in global impedance measurements. A separation between 2D and 3D distributions could be easily observed. In the case of a 2D distribution (AZ91 Mg alloy), the origin of the CPE behavior was the distribution of high-frequency resistance associated with the geometry of the disk electrode; whereas, the capacitance was independent of position. In the case of the aluminium electrode, the CPE behavior could be attributed to a combination of 3D and 2D distributions. Geometric distributions can play a significant role in the impedance response of electrochemical systems, and these distributions can lead to CPE behavior

Constant-Phase-Element Behavior Caused by Coupled Resistivity and Permittivity Distributions in Films

A recent proposed model showed that the impedance of a film with a uniform permittivity and a resistivity that varies along its thickness according to a power-law is in the form of a constant phase element (CPE). This model is further considered in order to assess the effect of non-uniform permittivity profiles. It is shown that a power-law permittivity profile is also compatible with a CPE behavior when resistivity and permittivity vary in opposite ways along the film thickness. This work shows that, for important classes of materials which show CPE behavior, relaxation of the assumption of a uniform permittivity does not alter the conclusions developed in the earlier work, and the formula relating film properties to CPE parameters is shown to apply

Oxygen transfer and bubble flow in split-cylinder airlift towers

Call number: LD2668 .T4 1978 O72Master of Scienc

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 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 origin of the complex character of the ohmic impedance

The local and global Ohmic response for an electrode exhibiting geometry-induced potential and/or current distributions has recently been shown to be represented by a frequency-dependent complex impedance. A physical explanation for this result is provided in terms of the radial contribution to local current density and the decrease in current density along the current lines. Experiments performed with Cu/Al and Mg/Al galvanic couples show that, in regions where a radial current density does not exist, the local Ohmic impedance is independent of position; whereas, in regions where the radial current density cannot be neglected, the local Ohmic impedance is a function of position. Simulations performed on recessed electrodes show that, even in the absence of a radial current, an axial variation of current density gives rise to a complex Ohmic impedance. The complex character of the Ohmic impedance shows that an equivalent circuit, using the usual two-terminal resistor to represent the Ohmic contribution of the electrolyte, provides an inadequate representation of an electrode with geometry-induced current and potential distributions

Local electrochemical impedance spectroscopy: A review and some recent developments

Local electrochemical impedance spectroscopy (LEIS), which provides a powerful tool for exploration of electrode heterogeneity, has its roots in the development of electrochemical techniques employing scanning of microelectrodes. The historical development of local impedance spectroscopy measurements is reviewed, and guidelines are presented for implementation of LEIS. The factors which control the limiting spatial resolution of the technique are identified. The mathematical foundation for the technique is reviewed, including definitions of interfacial and local Ohmic impedances on both local and global scales. Experimental results for the reduction of ferricyanide show the correspondence between local and global impedances. Simulations for a single Faradaic reaction on a disk electrode embedded in an insulator are used to show that the Ohmic contribution, traditionally considered to be a real value, can have complex character in certain frequency ranges

Tribute to Bernard Tribollet

The authors of this foreword started their collaboration with Bernard Tribollet many years ago. The dates of the earliest co-signed papers are 1988 for Nadine Pébère and Marco Musiani, 1992 for Oscar Mattos, 1996 for Mark Orazem, and 2005 for Vincent Vivier, the youngest person in our group. The fact that there were joint publications ever since testifies to how interesting and rewarding working with Bernard has been for us all

Enhanced Graphical Representation of Electrochemical Impedance Data

Bode plots, corrected for Ohmic resistance, logarithmic plots of the imaginary component of the impedance, and effective capacitance plots are shown to be useful complements to the more traditionally used complex-plane and Bode representations for electrochemical impedance data. The graphical methods are illustrated by synthetic data and by experimental data associated with corrosion in saline environments. Bode plots are shown, in particular, to be confounded by the influence of electrolyte resistance. The plots proposed here provide useful guides to model development for both reactive and blocking systems. The logarithmic plots of the imaginary component of the impedance and effective capacitance plots are useful for all impedance data, and the correction for Ohmic resistance in Bode plots is useful when the solution resistance is not negligible

Impedance analysis of the distributed resistivity of coatings in dry and wet conditions

A commercial coating (epoxy-polyaminoamide waterborne paint) deposited on a 2024 aluminium alloy was characterized by impedance measurements, first in dry conditions and then as a function of the immersion time in NaCl solutions (wet conditions). The behaviour of the dry coating was close to that of an ideal capacitor and could be accurately modelled with the power-law model corresponding to a constant phase element (CPE) behaviour. Upon immersion in NaCl solutions, the behaviour of the wet coating became progressively less ideal, i.e. farther from a capacitive behaviour. This result provided support to the hypothesis that an inhomogeneous uptake of the electrolyte solution was the cause of the often observed non-ideal responses of wet coatings. The experimental EIS data recorded for immersion times up to 504 hours were compared with models assuming either a power-law or an exponential variation of the coating resistivity along its thickness, respectively implying a phase angle independent of frequency or slightly dependent on it