Enhanced Graphical Representation of Electrochemical Impedance Data

Abstract

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. 12 Graphical methods provide the first step toward interpretation and evaluation of impedance data. A number of authors have described graphical methods based on a deterministic model for a given process. Mott-Shottky plots of 1/C sc 2 as a function of potential are used to obtain flatband potentials and doping levels characteristic of semiconducting systems.1 A graphical method was reported by Tribollet et al. that can be used to extract Schmidt numbers from experimental data in which the convective diffusion impedance dominates.2 The technique accounts for the finite value of the Schmidt number. Graphical methods in terms of a dimensionless frequency scaled by rotation speed are commonly used for the interpretation of electrohydrodynamic impedance measurements.3 Jansen and Orazem describe a graphical superposition of impedance data collected at different temperatures that reveals the influence of a single dominant activation-energy controlled process in solid-state systems.4 When a specific model is not postulated, typically two types of graphical representations are used: complex-impedance-plane plots, often called Nyquist plots, and Bode plots. [5][6][7] The Bode representation consists of the logarithm of impedance magnitude and the phase angle, both plotted as a function of frequency on a logarithmic scale. These types of representation have become the standard in impedance analysis for any complex quantity, e.g., admittance, complex capacitance, and electrohydrodynamic impedance, as well as electrochemical impedance.The object of this work is to demonstrate the limitations of the usual methods for representing impedance data and to suggest some useful alternatives. The proposed graphical methods are illustrated both by synthetic data and by experimental data associated with corrosion of a magnesium alloy AZ91 in saline environments. Synthetic DataThe methods for graphical representation and interpretation of electrochemical impedance are presented here for data characteristic of active and blocking electrodes. The classification of active systems is represented by a Randles circuit, as presented in Fig. 1a. The classification of blocking systems is represented ...

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