POLYMER-FILMS ON ELECTRODES .26. STUDY OF ION-TRANSPORT AND ELECTRON-TRANSFER AT POLYPYRROLE FILMS BY SCANNING ELECTROCHEMICAL MICROSCOPY

The oxidation and reduction of the conductive polymer polypyrrole (PPy ) doped with different counterions (bromide, ferrocyanide (FCN) and po ly(p-styrenesulfonate)) was studied by scanning electrochemical micros copy (SECM) during both potential step (chronocoulometric) and cyclic voltammetric scans. The ultramicroelectrode tip was positioned close t o the surface of a PPy-modified substrate electrode, and the responses of both electrodes to a substrate potential step or linear sweep were monitored simultaneously. In this way, the rates of bromide or ferroc yanide ejection during PPy reduction were shown to be functions of the reduction potential. The nature of the cation (e.g., TBA(+) vs K+) wa s an important factor determining the kinetics of ion transport in the PPy(+)/FCN- films. With a solution containing TBA(+) the release of F e(CN)(6)(4-) during PPy(+)/FCN- reduction was kinetically slow, and so me ferrocyanide-containing species were also ejected during film reoxi dation. Direct evidence for the incorporation of cations (e.g., Ru(NH3 )(6)(3/2+)) in a PPy film during its reduction was also obtained by SE CM measurements. Finally, the physical localization and mechanism of F c(+) (Fc = ferrocene) and Os(bpy)(3)(3+) (bpy = 2,2'-bipyridine) reduc tion at oxidized and reduced PPy are discussed in connection with the applicability of different models for conducting polymers.