Investigation of Electrochemical Processes at Electrodes

Numerical simulations reveal that this effective value Rif differs form Re if the measured (simulated) Rif is smaller than Rsw (Fig. 21a). These deviations are connected with the inhomogeneous potential distribution in the vicinity of a microelectrode yielding laterally varying electrode overvoltages for small electrode resistances (Fig. 21b). In microelectrode experiments, however, the ratio Reef / Rm is often large (cf. Sec. 3.4) and hence Ra x R f is usually a reasonable approximation. In liquid electrochemistry, similar effects are discussed in the context of primary (Rei = 0) and secondary (Re 0) current distributions [268, 269]. 

In the dc case, the current has to flow to the electrochemically active sites close to the 3PB and a considerable current constriction results (Fig. 22b). Hence, the dc resistance consists of the resistance due to the electrochemical reaction and the bulk resistance, which includes the current constriction in the vicinity of the active ring. At higher frequencies, the inner part of the microelectrode becomes dielectrically permeable and the current flows to the entire microelectrode area (Fig. 22c). In other words, the current lines in the bulk, and thus the bulk resistance, are frequency- 

Numerical calculations of several impedance spectra confirmed that a fit to the intuitive circuit (Fig. 23b) really yields sensible approximations to the expected parameters (cf. Ref. [53]). The circuit can therefore be applied to analyze such microelectrode experiments and particularly to separate the true electrochemical im- 

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