CHARGING PROCESS IN POLYPYRROLE FILMS - EFFECT OF ION ASSOCIATION

Cyclic voltammetric (CV) response of polypyrrole (PPy) films in contac t with tetraphenylborate (TPB) acetonitrile (AN) solutions has demonst rated pronounced anomalies compared to the CV for the usual anions. Th e redox activity of the film is considerably reduced and both the anod ic and cathodic waves consist of two peaks whose positions are strongl y dependent on the ionic strength of the solution. The diminution of t he charging ability has been related to a much lower maximum concentra tion of the doping anions (TPB) inside the film due to their large siz e. The observed splitting of the waves into two peaks cannot be attrib uted to two different electronic processes like the successive generat ion of 'polarons' and then 'bipolarons'. On the other hand, the hypoth esis that ions inside the film may be in two different forms,'free' an d '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 l ogarithm of the ionic strength, independence of the peak heights or sh apes of the concentration, merging of the peaks at higher ionic streng ths. The same concept has been able to explain an observed separation of the broad peak for polyaniline (PANI) films in contact with mixed p erchlorate-polyanion solutions into two narrower peaks upon diminishin g the perchlorate content. This conclusion is in conformity with our a nalysis of data for the charging process of PPy in perchlorate solutio ns obtained earlier by in situ electrochemical quartz-crystal microbal ance studies (EQCM). Experimental plots for mass versus its charge hav e quite different shapes for films polymerised at high or low potentia ls. In particular, those curves in the latter case possess opposite sl opes 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 p hase. The comparison has also provided evidence that 'bound' ions can be exchanged gradually with their 'free' form during the cyclic voltam metry process. This process results in a complicated shape for the mas s 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 neut ral sites of the polymer matrix, either due to the formation of a bond , or the ion binding by micro-cavities. An alternative approach assume s the anions to form a complex with positively charged electronic spec ies (like 'polarons' or 'bipolarons'). However, the latter is unable e ven to provide a qualitative explanation of the above data, e.g. oppos ite signs of the slope for low and high charge ranges. (C) 1998 Elsevi er Science S.A. All rights reserved.