Electroactive bilayers employing conducting polymers. Part 6. Kinetic electrochemical quartz crystal microbalance measurements

During the electrochemical reduction and oxidation of polybithiophene/polyxylylviologen (PBT/PXV) bilayers, background electrolyte ions move into and out of the film in order to maintain overall charge neutrality within the film. Since the electrolyte contains two mobile ions of opposite charge (tetraethylammonium and perchlorate), in general, charge neutrality can be obtained by either the influx of one, or the efflux of the other. The comparison of electrochemical quartz crystal microbalance (EQCM) mass measurements made on fast and slow timescales, together with variation in the inner PBT and outer PXV film thicknesses, has been used to evaluate which ions transfer from the solution to the film at different stages of reduction and oxidation. These measurements show that the predominant ion motion is ClO4− in response to both reduction and reoxidation of the PXV layer. However, at both the onset of PXV2+ reduction and the onset of PXV0 reoxidation, a small number of TEA+ ions move. Furthermore, the different film transport rates of ClO4− and TEA+ mean that a rapid change in electrode potential coupled with a thick inner PBT layer can lead to the transient accumulation of electrolyte within the film. These observations lead to a mechanism accounting for charge and mass trapping and for rectification effects, and show that even minor inner layer redox conversion to a doped state can open up efficient charge transport pathways from the underlying electrode to the outer polymer layer.