THE AC IMPEDANCE OF A 3-ION THIN-LAYER CELL

A thin layer cell containing three ions M+, B+ and X- is considered where M+ is electroactive while B+ and X- are electrochemically inert. A transmission line model with a resistance for each of the three ions is used to model the cell. The response to a small step in potential is found and hence the ac impedance function. The results are presented as a function of the two resistance ratios and of the fraction of M+ present. The results for the two limiting cases of tracer diffusion of M+ in background electrolyte and of a binary M+X- electrolyte agree exactly with previous treatments. A case diagram is constructed to describe the results for the intermediate cases. In some cases, impedance plots with two humps can be found.