J. Tanguy

We have studied the behavior of a chemically synthesized polypyrrole as electrode material. The different electrochemical methods that we have used for this characterization, namely, cyclic voltammetry, ac impedance measurements, and cyclic chronopotentiometry, have evidenced the role of the low frequency capacitance characteristic of the doped state of polypyrrole. This capacitance seems independent of the synthesis conditions of the polymer, and we show that it partially explains the large value of the total charge stored in this polypyrrole. Analyzing the effect of residual water in the polymer, we show that a water content of about 3% (w/w) increases the total charge and lowers the redox potential . A quasi linear relationship has been found between and , the massic capacity: increases when decreases. This relation can be also attributed to the charging of the capacitance , where is the degradation potential. We discuss the origin of this capacitance .

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 oxidized state. The nature and the origin 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.

Hydrophobic polypyrrole (PPy) coatings on zinc electrodes were prepared by a one-step electrochemical method. The process is ultrafast; 2 µm of PPy can be deposited in less than 3 s. It is thus compatible with industrial requirements for the coating of large surfaces for various industries (automotive, buildings, ships). The coatings obtained have apparent contact angles as high as 125° and can thus be used as cheap highly hydrophobic surfaces. They have been fully characterized using various physicochemical techniques (EQCM, XPS) and their anticorrosion properties have been tested. It is found that postpolymerization heat treatment makes it possible to reach a situation where 1 µm of polypyrrole is equivalent to 1 µm of a zinc coating for preventing the corrosion of mild steel.