Influence of ionic size on the mechanism of electrochemical doping of polypyrrole 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 acetonitrile with a comparison to that in perchlorate solutions. The overall redox activity in the former case is significantly reduced. Nevertheless, we have been able to give a clear cyclic-voltammetry characterization of the film at various bulk-electrolyte concentrations for different sweeping rates. Those data reveal two well separated waves in each potential scan direction located in the 0.1 M NaBPh₄ solution at -0.38 V and -0.1 V (vs Ag/AgBPh₄ 0.1 M in CH₃CN) in the anodic branch. The first oxidation wave obeys an ultra-thin layer mass transfer mode (temporal mode), while the second one is diffusional for sufficiently high scan rates but it approaches the same quasi-equilibrium charging regime at lower sweeping rates. Decrease of the bulk electrolyte concentration leads to a shift of both peaks in opposite directions from the merging point, keeping their shapes and intensities unchanged, with the positive or negative 60 mV slope in the semi-logarithmic coordinates. Both peaks have been attributed to a single redox transition with participation of single-charged electronic species but with different mechanisms of the charge compensation by ions. Splitting the curve into two peaks is assumed to be due to the existence of two different forms 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 removed from the file during the first step of the charging process. The second redox wave is related to the free-anion insertion into the film. This approach is able to reproduce properly all features of experimental data.