Application to a Randles Circuit

The pareimeter values used for the simulations presented here are given in Table 17.1. The parameters were chosen such that the high-frequency CPE element would have a characteristic frequency /rc of 100 Hz, corresponding to a time constant of 1.59 ms. 

The Randles circuit provides an example of a class of systems for which, at the zero-frequency or dc limit, the resistance to passage of current is finite, and current can pass. Many electrochemical and electroiuc systems exhibit such nonblocking or reactive behavior. Even though the impedance response of the system presented in this section is relatively simple as compared to that of t37pical electrochemical and electronic systems, the nonblocking systems comprise a broad cross-section of electrochemical and electronic systems. The concepts described in this chapter therefore can be easily adapted to experimental data. 

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The diffusion impedance at semiconductor electrodes has been considered recently [105]. This author described the applicability of AC impedance spectroscopy for the study of electron capture and hole injection processes at n-CaAs-H20/C2H50H-methyl viologen, p-InP-aq. KOH-Fe(CN)6l -GaAs-H2S04-Ce +, and -InP-aq. KOH-Fe(CN)6 interfaces. In the case of electron capture processes, a Randles-like equivalent circuit was found to be applicable [105]. On the other hand, no Warburg component was present in the hole injection case when the reverse... 

An ideal electrode-electrolyte interface with an electron-transfer process can be described using Randle equivalent circuit shown in Fig. 2.7. The Faradaic electron-transfer reaction is represented by a charge transfer resistance and the mass transfer of the electroactive species is described by Warburg element (W). The electrolyte resistance R is in series with the parallel combination of the double-layer capacitance Cdi and an impedance of a Faradaic reaction. However, in practical application, the impedance results for a solid electrode/electrolyte interface often reveal a frequency dispersion that cannot be described by simple Randle circuit and simple electronic components. The interaction of each component in an electrochemical system contributes to the complexity of final impedance spectroscopy results. The FIS results often consist of resistive, capacitive, and inductive components, and all of them can be influenced by analytes and their local environment, corresponding to solvent, electrolyte, electrode condition, and other possible electrochemically active species. It is important to characterize the electrode/electrolyte interface properties by FIS for their real-world applications in sensors and energy storage applications. 

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