Ion and solvent transfers accompanying redox switching of polypyrrole films immersed in divalent anion solutions

Electrochemical quartz crystal microbalance (EQCM) data acquired under cyclic voltammetric conditions were used to investigate the interfacial mass transfers accompanying redox switching of polypyrrole (PPy) films immersed in aqueous solutions of divalent anions. In contrast to the dominance of anion transfer in univalent anion (perchlorate) solution, PPy redox switching in 0.1 M Na2SO4, 0.1 M Na2C2O4, 0.1 M Na2HPO4 and 0.1 M Na2CO3 solutions involves mixed anion/cation transfers, with the cation as the dominant species. Cation transfer is accompanied by water transfer, to the extent of 0.5–3 mol of water per mole of cation; in part, solvent transfers as a result of a combined osmotic and a volume constraint. Almost all the injected charge is recovered during PPy redox switching in a given dianion electrolyte, but the EQCM responses evolve with repetitive film redox switching. There is a redox-induced increase in the reduced film's mass. Over the course of a few cycles (the exact number depending on electrolyte and film thickness), the potential dependences of the film charge and mass responses achieve a steady state significantly different from that for an equilibrated undoped film.