Anomalously large current plateaus observed in the cyclic voltammograms of some conducting polymers such as polypyrrole have been recently interpreted as the charging of a large capacitance taking place in the polymer in the oxxdized state. The nature and the orxgin of such capacitance is actually discussed either in terms of a static charge associated with a double layer formation in the porous electrode, or in terms of an overdoping of the polymer in a non-Nernstian redox process. In order to clarify this situation we made a systematic electrochemical study by cyclic voltametry, ac impedance measurements, and charging-discharging cycles on some chemically synthesized polypyrrole samples. Analyzing the response to the ac signal as a function of frequency, it appears that the capacitance effect can be described in terms of an ionic relaxation mechanism. A large part of the ions involved in the electrochemical doping exhibits a relatively short relaxation time, follow the low frequency ac signal, and are responsible for the capacitance effect. Another part of the doping ions appears to be deeply trapped in the polymer chain: they do not follow the ac signal, and do not participate in the capacitance effect. In this way we have identified two types of currents in the cyclic voltammetry: a "capacitive current" without hysteresis effect, and a "noncapacitive current" arising from the deeply trapped ions giving a large hysteresis responsible for the broadening of the reduction peak. From those results we have derived a model for the electrochemical doping assuming the existence of two types of ionic trapping sites in the polymer chains, and the capacitive effect is analyzed as a contribution to the doping process itself.

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