Kinetics of heterocyclic polymers

Considerable attention is presently devoted to heterocyclic polymers, such as polypyrrole, polythiophene and their derivatives. The kinetics of the electrochemical doping processes of these polymers has been extensively studied in electrochemical cells using non-aqueous electrolytes. 

These studies have been mainly carried out using cyclic voltammetry and frequency response analysis as experimental tools. As a typical example. Fig. 9.12 illustrates the voltammogram related to the p-doping process of a polypyrrole film electrode in the LiClQ -propylene carbonate electrolyte, i.e. the reaction already indicated by (9.16). 

Two main features may be identified by examining the trend of this typical voltammogram namely  

Further information on this subject can be obtained by frequency response analysis and this technique has proved to be very valuable for studying the kinetics of polymer electrodes. Initially, it has been shown that the overall impedance response of polymer electrodes generally resembles that of intercalation electrodes, such as TiS2 and WO3 (Ho, Raistrick and Huggins, 1980 Naoi, Ueyama, Osaka and Smyrl, 1990). On the other hand this was to be expected since polymer and intercalation electrodes both undergo somewhat similar electrochemical redox reactions, which include the diffusion of ions in the bulk of the host structures. One aspect of this conclusion is that the impedance response of polymer electrodes may be interpreted on the basis of electrical circuits which are representative of the intercalation electrodes, such as the Randles circuit illustrated in Fig. 9.13. The figure also illustrates the idealised response of this circuit in the complex impedance jZ -Z ) plane. 

At low frequencies the impedance is dominated by diffusion. Two regions may be identified in the complex impedance, a linear region with a phase angle of ti/4 corresponding to semiinfinite diffusion and 

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