Journal of Electroanalytical Chemistry, Vol.450, No.1, 121-141, 1998
Charging process in polypyrrole films : effect of ion association
Cyclic voltammetric (CV) response of polypyrrole (PPy) films in contact with tetraphenylborate (TPB) acetonitrile (AN) solutions has demonstrated pronounced anomalies compared to the CV for the usual anions. The redox activity of the film is considerably reduced and both the anodic and cathodic waves consist of two peaks whose positions are strongly dependent on the ionic strength of the solution. The diminution of the charging ability has been related to a much lower maximum concentration of the doping anions (TPB) inside the film due to their large size. The observed splitting of the waves into two peaks cannot be attributed to two different electronic processes like the successive generation of 'polarons' and then 'bipolarons'. On the other hand, the hypothesis that ions inside the film may be in two different forms,'free' and 'bound', has allowed us to reproduce all the experimental features, in particular, two separated peaks at lower electrolyte concentrations, their shift in the opposite directions with +/- 60 mV slope versus logarithm of the ionic strength, independence of the peak heights or shapes of the concentration, merging of the peaks at higher ionic strengths. The same concept has been able to explain an observed separation of the broad peak for polyaniline (PANI) films in contact with mixed perchlorate-polyanion solutions into two narrower peaks upon diminishing the perchlorate content. This conclusion is in conformity with our analysis of data for the charging process of PPy in perchlorate solutions obtained earlier by in situ electrochemical quartz-crystal microbalance studies (EQCM). Experimental plots for mass versus its charge have quite different shapes for films polymerised at high or low potentials. In particular, those curves in the latter case possess opposite slopes at low and high film charges as well as a pronounced hysteresis. These observations have also been interpreted successfully within the framework of the above model of two forms of ions inside the polymer phase. The comparison has also provided evidence that 'bound' ions can be exchanged gradually with their 'free' form during the cyclic voltammetry process. This process results in a complicated shape for the mass versus charge plots including a crossing of the anodic and cathodic branches as well as a loop within the range of low charges. All these predictions of the model are in conformity with the above experimental results. The theory considers 'immobile' ions as associated with neutral sites of the polymer matrix, either due to the formation of a bond, or the ion binding by micro-cavities. An alternative approach assumes the anions to form a complex with positively charged electronic species (like 'polarons' or 'bipolarons'). However, the latter is unable even to provide a qualitative explanation of the above data, e.g. opposite signs of the slope for low and high charge ranges.
Keywords:QUARTZ CRYSTAL MICROBALANCE;ELECTROACTIVE POLYMER-FILMS;CONDUCTING POLYMERS;POLYANILINE FILMS;DOPED POLYPYRROLE;DODECYL-SULFATE;ELECTROCHEMICAL PROPERTIES;AQUEOUS-SOLUTIONS;ELECTRODES;TRANSPORT