Faradaic impedance reduction

Sluyters and coworkers [34] have studied the mechanism of Zn(II) reduction on DM E in NaCl04 solutions at different water activity (uw) using faradaic impedance method. Dqx and E p were determined from dc polarographic curves. Hydration numbers of Zn(Il) ion were estimated from the dependence of E[p on In Uw The obtained standard rate constant was changing with a NaCl04 concentration and the slope of the dependence of In k on potential was changing with potential (see Fig. 1). Therefore, the following mechanisms were proposed ... 

Fig. 4.7 Randles plot of faradaic impedance for reduction of nitromesitylene in dimethyl-formamide on a dropping mercury electrode, Zq - out-of-phase (imaginary), Zi in-phase (real) impedance (Reprinted with permission Ref. [151]. Copyright 1978 American Chemical Society)...

Figure 9,10 Nyquist plot of the Faradaic impedance Z .- of the oxygen reduction reaction simulated from the above reaction model and corresponding equivalent circuit. The model describes a competition between surface adsorption and incorporation, depending on the ratio of the rate constants for the respective processes.

A complication that occurs on a low at.% Ru electrode is that, owing to the low Faradaic currents (low Ru content) and hence large Rt value, currents due to other trace redox reactions, e.g. oxygen reduction, become more detectable. This reveals itself in a phase-angle of 45° as co 0 as trace oxygen reduction would be diffusion-controlled. The impedance corresponding to this situation can be shown to be the same as that in Equation 5.3, with U(p) expressed by the relationship ... 

An important conclusion from the paper by Brug et al. [1984] is that involvement of a CPE at solid electrodes used for studies of the impedance of Faradaic reactions can severely influence the frequency dispersion of interfacial admittance, leading to large errors in the determined Faradaic rate parameters. However, those authors note that it is feasible to account for the CPE effect correctly and to check the results of impedance analysis with respect to their internal consistency. The latter can be checked by a Kramers-Kronig analysis (cf. Lasia [1999]) which requires, however, detailed frequency-response data. Their approach was supported by experimental impedance studies on proton reduction at single and polycrystalline Au electrodes and on reduction of tris-oxalato-Fe(lII) (Brug et al. [1984]). 

More definite conclusions may be drawn on the basis of exhaustive analysis of impedance spectra that carry information not only on the kinetics of faradaic processes but also on the characteristics of a double electric layer. As for gluconate systems, such investigations are scarce. A great variety of Nyquist plots (relationships between real, and imaginary, components of impedance) are demonstrated in Ref [99] that deal with the deposition of tin from neutral gluconate baths, but no quantitative analysis is presented. As we established earlier [100, 101], Nyquist plots obtained at open-circuit potentials for surfactant-free solutions are nothing else than lines that were observed over an entire range of applied frequencies. This means that Sn(II) reduction is mainly controlled by diffusive mass transport. 

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