Electrochemical Properties and Electrochemical Impedance Spectroscopy of Polypyrrole-coated Platinum Electrodes

Polypyrrole (PPy) films of different thickness were characterized by electrochemical impedance spectroscopy (EIS) measurements in acetonitrile and aqueous solutions, containing 0.1 M NaClO4 or sodium dodecylsulfate as the dopant. The PPy films were electrochemically deposited on Pt, and their electrochemical properties studied by cyclic voltammetry. Impedance spectra were obtained at potentials ranging from 0 to 0.8 V/SCE. The EIS data were fitted using two different equivalent electrical circuits (depending on the nature of the dopant). They involve a diffusive capacitance, which increased with the passing charge during electrosynthesis (i.e. film thickness) for ClO4 -doped PPy, but was practically unaffected by the film thickness in the case of SDSdoped PPy. Also, a double-layer capacitance was found only in the circuit of ClO4 -doped PPy. It increased with the film thickness, and showed a minimum near the open-circuit potential. Finally the charge-transfer resistance (Rct) obtained with SDS is nearly 200-fold higher than that obtained with ClO4 in the same solvent (H2O). With the same dopant (ClO4 ), Rct is about five times higher in acetonitrile relative to water. All these EIS results of the different types of PPy suggest a relation with the wettability of the polymer.