IONIC EXCHANGE OF THE POLYPYRROLE FILM WITH THE PC LITHIUM PERCHLORATE SOLUTION DURING THE CHARGING PROCESS

Two theoretical approaches to the interpretation of the EQCM data for the charging-discharging process in electron-conducting polymer film c oated electrodes have been proposed, with their application to systems without attached charged groups, or fixed charges. The first model tr eats cations and anions inside the polymer phase as ''free'' mobile sp ecies which exchange with the solution may be retarded due to low valu es of the interfacial exchange constants. Depending on its value for t he anion transfer, three limiting regimes of the charging process have been found, (1) complete ionic equilibrium with the solution, (2) ''i rreversible anion transfer'', (3) ''cation compensation'', the electro nic charge in the two former regimes being dominantly balanced by coun ter-ions whereas the latter regime being the co-ion process. Theoretic al EQCM curves upon cyclic variation of the potential may possess a co mplicated shape, with a pronounced ''first cycle'' and ''relaxation'' effects as well as with a considerable hysteresis, the mass at the ano dic scan being greater than that at the cathodic branch, for the anion exchange constants between regimes 2 and 3. Another model proposes ex istence of two different forms of ions inside the film, ''free'' and ' 'bound'', with possible kinetic limitations for their mutual transform ation but a complete equilibrium between the ''free'' ions and the sol ution. Corresponding theoretical EQCM plots demonstrate a great variet y of shapes depending on the maximum amount and thermodynamical stabil ity of ''bound'' species as well a on the kinetic exchange parameters. This model has enabled one to reproduce most of the features of exper imental stabilized EQCM curves for PPy:non-monotonic variation of the film mass (''co-ion'' type at low charges and ''counter-ion'' one at h igher 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 lower mass at the cathodic scan at very low charges and a crossing po int of the branches. Copyright (C) 1996 Elsevier Science