Vicky Mei-Wen Huang

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.

Experimental Issues Associated with Measurement of Local Electrochemical Impedance

A detailed analysis from both numerical calculations and experimental results is presented for the local electrochemical impedance spectroscopy (LEIS) measurements of a disk electrode, which accounted for the placement of the LEIS probe above the electrode surface. The calculations for an electrode with a local constant phase element behavior were in excellent agreement with experimental observations for a stainless steel electrode in a Na 2 SO 4 /NaCl (pH 4) electrolyte. Measurement of the local impedance at two positions above the electrode surface allowed estimations of the local interfacial impedance and the local ohmic impedance, which were in excellent agreement with the simulations. The results demonstrate that the local impedance measurement itself cannot be used to estimate the surface area sampled by the LEIS probe. The area sampled must be determined instead by the probe geometry.

Erratum: The Apparent Constant-Phase-Element Behavior of an Ideally Polarized Blocking Electrode. A Global and Local Impedance Analysis [ J. Electrochem. Soc. , 154 , C81 (2007)]

Erratum: The Apparent Constant-Phase-Element Behavior of an Ideally Polarized Blocking Electrode. A Global and Local Impedance Analysis [J. Electrochem. Soc., 154, C81 (2007)] Vicky Mei-Wen Huang, Vincent Vivier, Mark E. Orazem, Nadine Pebere, and Bernard Tribollet Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611, USA UPR15 du CNRS, Laboratoire Interfaces et Systemes Electrochimiques, Universite Pierre et Marie Curie, 75252 Paris, France CIRIMAT, UMR CNRS 5085, ENSIACET, 31077 Toulouse Cedex 04, France