The behavior of polypyrrole-coated electrodes in propylene carbonate solutions. II. Kinetics of electrochemical doping studied by electrochemical impedance spectroscopy

A family of Nyquist plots for 0.2 μm thick polypyrrole film was measured in a wide range of potentials in 0.25 M LiClO₄/propylene carbonate (PC) solution. This film was galvanostatically deposited onto the surface of a Pt electrode, and the film was continuously cycled in 0.25 M LiClO₄/PC solution until a stable cyclic voltammetric response was reached. No irreversible changes occurred in the film during impedance measurements. Two alternative finite-space diffusion models were used to determine useful kinetic and equilibrium characteristics: the total charge-transfer resistance; partial electron and ion interfacial transfers; the low-frequency resistance related to the capacitive region of the impedance spectra; the binary chemical diffusion coefficient D; and the low-frequency differential capacitance related to the charging of the film's bulk. All these quantities were determined as functions of potential and adequately interpreted. We also found that upon increasing the doping level, D increases from 1.0 × 10^-9 to 2.8 × 10^-8 cm²/s.