DUAL TRANSMISSION-LINE WITH CHARGE-TRANSFER RESISTANCE FOR CONDUCTINGPOLYMERS

To describe charge conduction in conducting polymers a dual transmissi on line with one resistance describing electronic motion in the polyme r and a second resistance describing ionic motion in the pores has pro ved applicable to a number of systems. A further feature of ac impedan ce plots for these polymers is a high-frequency semicircle. This may b e attributed to a charge-transfer resistance and double-layer capacita nce at either the electrode/polymer or the polymer/electrolyte interfa ce. A general expression for such a charge-transfer resistance is deri ved and it is shown that the current will be proportional to the diffe rence in Nernst potential across such a resistance. This circuit eleme nt together with a double-layer capacity is then combined with the dua l transmission line. A general expression for the ac impedance for the combined circuit is obtained. Care has to be taken to use the right p otentials, Nernst, Donnan or the actual potential. Depending on the re lative resistances in the transmission line different types of behavio ur can be found. In particular the variation of the real part of the i mpedance at high frequency can be used to determine whether the charge -transfer resistance is at the electrode or at the electrolyte interfa ce. The procedure is exemplified with results from polypyrrole.