Warburg species differences

Gerischer impedance — The Gerischer impedance is a transport-related interfacial impedance element which differs from the Warburg impedance in that the electroactive species taking part in the electrode process is chemically generated in a spatially homogeneous way prior to diffusing to the interface. It has the form ... 

Fig. 14.7 Electrical equivalent model of mechanism involving one adsorbed species with diffusion (a) and a model with different position of Warburg element (b)...

In solid electrolytes, however, the unsupported electroactive species is often the sole charge carrier. It is thus impossible to change the interfacial potential difference without changing the concentration of ions in the double-layer region. This means of course, that the normal Warburg impedance is not seen, but it also means that there is a coupling between the faradic current and the double-layer charging. 

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