Bode plots, interpretation

There are two possibilities. They are shown in Fig. 8.4, together with their interpretations on a Bode plot. 

First, we need to generate the plots. Use Fig. E8.12 to help interpret the MATLAB generated Bode plot.1... 

Caution should be used when interpreting the electrolyte-resistance-corrected Bode plots. As seen in equation (16.20), nonzero values for Rg — Rg t) can give the appearance of an additional high-frequency relaxation process. When possible, an eissessment of Rg i should be made independently of the regression. 

Intuitive interpretation of experiences. One of the direct applications of the relation between the value of the evolution mode and the behavior of energy is that an immediate diagnosis can be made without a computer, by hand, of what happens in the system under study. The phase angle measured by the slope of a Nyquist (Cole-Cole) plot or a Bode plot gives directly the proportion of conserved energy during the process for a given frequency. 

In order to correctly interpret the experimental information provided by EIS, conveyed in either Nyquist or Bode plots, the use of a sound physical model describing the relevant biophysicochemical processes taking place in the system is essential. A simple strategy to deal with the experimental information involves the implementation of the model into an equivalent circuit, which contains all the information of charge transport. In the equivalent circuit, the resistances and capacitances describe the charge loss and accumulation mechanisms that can take place in the system. In the following section, we first describe the most common circuit elements used in EIS data analysis, followed by the most common equivalent circuits used to describe typical electrochemical cells. 

Rt represents polarization resistance and Q, is the double layer capacitance. In this case, Nyquist and Bode plots depict only a time constant. However, in most real systems the plots represent deviations from this response leading to a more difficult interpretation of the impedance spectra. 

Alternatively, an equally powerful visualization of impedance data involves Bode analysis. In this case, the magnitude of the impedance and the phase shift are plotted separately as functions of the frequency of the perturbation. This approach was developed to analyze electric circuits in terms of critical resistive and capacitive elements. A similar approach is taken in impedance spectroscopy, and impedance responses of materials are interpreted in terms of equivalent electric circuits. The individual components of the equivalent circuit are further interpreted in terms of phemonenological responses such as ionic conductivity, dielectric behavior, relaxation times, mobility, and diffusion. 

Impedance data may be analyzed directly from impedance plots (Bode and Nyquist plots). Alternatively, the capacitances and the resistances are varied until the model predicted ac behavior matches the experimental data using a circuit analog model. These electrical properties are then interpreted as the properties of the system. 

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