Mechanism and kinetics of the electrochemical formation of polypyrrole under forced convection conditions

This work studies the process of electrochemical formation of polypyrrole onto a vitreous carbon rotating disk electrode, from a pyrrole aqueous solution containing sodium nitrate, applying different potentials to the electrode for both conditions: stagnant and forced convection. It was found that the analyses of the experimental current transients, in both cases, could be done using the same theoretical model. The latter involves three different contributions: a first one due to pyrrole oxidation on the vitreous carbon surface, to form different oligomers in solution, a second one related with bidimensional nucleation of the oligormeric form of pyrrole (formed during the first stage) that saturated the solution near the electrode surface and a final contribution due to oxidation of pyrrole and/or its oligomers onto the surface of the polypyrrole film deposited during the 2D nucleation. The last contribution can be related to the polypyrrole three dimensional growth through direct incorporation of the monomer to the polymeric film or to a similar processes as described on the first two stages. For the stagnant solution the contribution related with the third stage was the highest observed, for a given applied potential, however, when hydrodynamic conditions were imposed, at a constant potential, the contribution due to this stage diminishes with the angular speed of the RDE electrode, because the more reactive oligomeric forms of pyrrole are forced away from the electrode surface, to practically reach a steady value. From AFM images it is shown that toroidal structures became apparent when deposition was done potentiodynamically and that the whole electrode surface was covered by small nuclei which possess an average height that is quite similar to that theoretically predicted by the deposition model here proposed to explain the potentiostatic Ppy electrodeposition.