Ionic exchange of the polypyrrole film with the PC lithium perchlorate solution during the charging process

Abstract

Two theoretical approaches to the interpretation of the EQCM data for the charging-discharging process in electron-conducting polymer film coated electrodes have been proposed, with their application to systems without attached charged groups, or fixed charges. The first model treats cations and anions inside the polymer phase as “free” mobile species which exchange with the solution may be retarded due to low values of the interfacial exchange constants. Depending on its value for the anion transfer, three limiting regimes of the charging process have been found, 1. (1) complete ionic equilibrium with the solution, 2. (2) “irreversible anion transfer”, 3. (3) “cation compensation”, the electronic charge in the two former regimes being dominantly balanced by counter-ions whereas the latter regime being the co-ion process. Theoretical EQCM curves upon cyclic variation of the potential may possess a complicated shape, with a pronounced “first cycle” and “relaxation” effects as well as with a considerable hysteresis, the mass at the anodic scan being greater than that at the cathodic branch, for the anion exchange constants between regimes 2 and 3. Another model proposes existence of two different forms of ions inside the film, “free” and “bound”, with possible kinetic limitations for their mutual transformation but a complete equilibrium between the “free” ions and the solution. Corresponding theoretical EQCM plots demonstrate a great variety of shapes depending on the maximum amount and thermodynamical stability of “bound” species as well as on the kinetic exchange parameters. This model has enabled one to reproduce most of the features of experimental stabilized EQCM curves for PPy: non-monotonic variation of the film mass (“co-ion” type at low charges and “counter-ion” one at higher charges), with a drastic change of the mass vs. charge slope at the cathodic scan and a flat extremum in the anodic branch, as well as a lower mass at the cathodic scan at very low charges and a crossing point of the branches.