Comparison of the ac Impedance of Conducting Polymer Films studied as Electrode-Supported and Free-Standing Membranes

The ac impedance response of a symmetrically bathed membrane consisting of a conducting polymer is modeled. The model is an extension of a recent one proposed for a conducting polymer film contacting an electronic and an ionic conductor on its two faces (modified electrode geometry) and takes into account both diffusion-migration of ionic and electronic charge carriers inside the film and charge-transfer across metal/polymer and polymer/electrolyte interfaces. The membrane impedance is calculated for various combinations of parameters in order to stress the effect of diffusion coefficients, charge-transfer resistance, and double layer capacitance at the interfaces. The impedance responses in the modified electrode and membrane geometries are compared. It is thus shown how some ambiguities in the attribution of certain features of the impedance of modified electrodes to either the polymer/electrode or polymer/electrolyte interfaces may be eliminated by this comparison.