Charge transfer impedance

In our opinion, the interesting photoresponses described by Dvorak et al. were incorrectly interpreted by the spurious definition of the photoinduced charge transfer impedance [157]. Formally, the impedance under illumination is determined by the AC admittance under constant illumination associated with a sinusoidal potential perturbation, i.e., under short-circuit conditions. From a simple phenomenological model, the dynamics of photoinduced charge transfer affect the charge distribution across the interface, thus according to the frequency of potential perturbation, the time constants associated with the various rate constants can be obtained [156,159-163]. It can be concluded from the magnitude of the photoeffects observed in the systems studied by Dvorak et al., that the impedance of the system is mostly determined by the time constant. 

Flash Rusting (Bulk Paint and "Wet" Film Studies). The moderate conductivity (50-100 ohm-cm) of the water borne paint formulations allowed both dc potentiodynamic and ac impedance studies of mild steel in the bulk paints to be measured. (Table I). AC impedance measurements at the potentiostatically controlled corrosion potentials indicated depressed semi-circles with a Warburg diffusion low frequency tail in the Nyquist plots (Figure 2). These measurements at 10, 30 and 60 minute exposure times, showed the presence of a reaction involving both charge transfer and mass transfer controlling processes. The charge transfer impedance 0 was readily obtained from extrapolation of the semi-circle to the real axis at low frequencies. The transfer impedance increased with exposure time in all cases. 

At 60 minutes only, dc potentiodynamic curves were determined from which the corrosion current was obtained by extrapolation of the anodic Tafel slope to the corrosion potential. The anodic Tafel slope b was generally between 70 to 80 mV whereas the cathodic curve continuously increased to a limiting diffusion current. The curves supported impedance data in indicating the presence of charge transfer and mass transfer control processes. The measurements at 60 minutes indicated a linear relationship between and 0 of slope 21mV. This confirmed that charge transfer impedance could be used to provide a measure of the corrosion rate at intermediate exposure times and these values are summarised in Table 1. 

The above formulas may become inapplicable for systems with adsorption processes or/and coupled chemical steps in solution whose characteristic times are comparable with the inverse frequency within the impedance measurement interval. In this case the charge-transfer resistance, Rct, must be replaced by a complex charge-transfer impedance, Zct. Another restriction of this treatment is its assumption of the uniform polarization of the m s interface which requires to ensure a highly symmetrical configuration of the system. Refs. [i] Sluyters-Rehbach M, Sluyters JH (1970) Sine wave methods in the study of electrode processes. In Bard A/ (ed) Electroanalytical chemistry, vol. 4. Marcel Dekker, New York, p 1 [ii] Bard A], Faulkner LR (2001) Electrochemical methods, 2nd edn. Wiley, New York [iii] Retter U, Lohse H (2005) Electrochemical impedance spectroscopy. In Scholz F (ed) Electroanalytical methods. Springer, Berlin, pp 149-166 [iv] Bar-soukov E, Macdonald JR (ed) (2005) Impedance spectroscopy. Wiley, Hoboken... 

The dependence of the mass-transfer and charge-transfer impedances on the electrode potential is displayed in Fig. 12. The charge-transfer and mass-transfer impedances have a minimum at Es [Eq. (68)] and Ey2, respectively, according to Eqs. (67) and (69). 

The fuel cell cathode can be represented by a series electrolyte resistance, a parallel double-layer capacitance, a charge transfer impedance, and finite Warburg impedance for diffusion process. This circuit model polarization is due to a combination of kinetic and diffusion processes. The Nyquist plot for this shows a semicircle with a 45° straight line. 

If the capacitive current could be neglected, the impedance would be determined by the charge transfer impedance plus the ohmic resistance in the electrolyte, and a plot of the real... 

The charge transfer impedance shows strong dependence on the difference between the applied electrochemical DC potential V and the "standard redox reaction potential" When V = V the electrochemical charge transfer between... 

The impedance for an absorbing or transmitting boundary where concentration of species outside of the diffusion layer at distance X from the electrode surface is constant as C(X >L, t) = C and the species are instantly consumed at the electrode surface as C (X = L, t) = 0 [1, p. 59, 3]. The situation can be represented by negligible charge-transfer impedance where Z j(o) = 0, and Eq. 5-45 becomes [36] ... 

Other more complicated forms of impedance-frequency dependencies can be revealed when a combination of diffusion process and homogeneous bulk solution reaction or recombination of electroactive species is considered [34]. In the case of homogeneous bulk solution and electrochemical reaction, the corresponding charge-transfer impedance Z can be simplified by a kinetic resistance R. Another parameter, —critical frequency of reaction or recom-... 

Figure 13 shows typical complex impedance spectra of the three types of polymer electrolytes sandwiched between lithium electrodes at an oscillation level of 0.5 V. The profiles of the spectra were two neighboring arcs. The low-frequency arcs (right-hand side arcs) and the high-frequency arcs (left-hand side arcs) corresponded to the loci of the charge transfer impedance and the bulk electrolyte impedance, respectively. The bulk resistance (/ .0 and the charge transfer resistance (R ) are shown in Fig. 13. The ionic conductivities at 50°C for the polymer electrolytes (1), (2), and (3) were approximately 10 to 10 S cm (depending on the LiC104 concentration (see Table 1), 10 S cm, and 10 S cm , respectively. The values, corresponding to the electrode reaction, were about lO" Cl, irrespective of the kinds of polymer electrolytes. These values were similar to those found in the poly(propylene oxide) networks with dissolved lithium salts [93] and to those found in the poly(ethylene succinate) with dissolved lithium salts [88,94], but were considerably higher than those found in poly(p-propiolactone)-LiC104 complexes [95].

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