INFLUENCE OF IONIC SIZE ON THE MECHANISM OF ELECTROCHEMICAL DOPING OFPOLYPYRROLE FILMS STUDIED BY CYCLIC VOLTAMMETRY

Charging/discharging process of a polypyrrole film has been studied in contact with a large-size anion (tetraphenylborate) solution in aceto nitrile with a comparison to that in perchlorate solutions. The overal l redox activity in the former case is significantly reduced. Neverthe less, we have been able to give a clear cyclic-voltammetry characteriz ation of the film at various bulk-electrolyte concentrations for diffe rent sweeping rates. Those data reveal two well separated waves in eac h potential scan direction located in the 0.1 M NaBPh(4) solution at - 0.38 V and -0.1 V (vs Ag/AgBPh(4) 0.1 M in CH3CN) in the anodic branch . The first oxidation wave obeys an ultra-thin layer mass transfer mod e (temporal mode), while the second one is diffusional for sufficientl y high scan rates but it approaches the same quasi-equilibrium chargin g regime at lower sweeping rates. Decrease of the bulk electrolyte con centration leads to a shift of both peaks in opposite directions from the merging point, keeping their shapes and intensities unchanged, wit h the positive or negative 60 mV slope in the semi-logarithmic coordin ates. Both peaks have been attributed to a single redox transition wit h participation of single-charged electronic species but with differen t mechanisms of the charge compensation by ions. Splitting the curve i nto two peaks is assumed to be due to the existence of two different f orms of ionic species inside the polymer phase, ''free'' and ''bound'' ones. Presence of some amount of bound anions in the reduced state is accompanied by the corresponding concentration of cations which are r emoved from the file during the first step of the charging process. Th e second redox wave is related to the free-anion insertion into the fi lm. This approach is able to reproduce properly all features of experi mental data. Copyright (C) 1996 Elsevier Science Ltd.