The roles of anion and solvent transport during the redox switching process at a poly(butyl viologen) film studied by an EQCM

In this study, three electrolytes (KCl, LiCl, and KNO3, each at 0.5 M in aqueous solution) were chosen to study the ion and solvent effect on the redox performance of poly(butyl viologen) (PBV) thin-films between its di-cation and radical-cation state, which is referred as its first redox couple. Before considering the role of ionic transport on the redox process, the exchange between ferrocyanide and anion should be completed. Since the deposition solution of PBV contains potassium ferrocyanide, the residual ferrocyanides inside the films would be exchanged by smaller anions from the bulk solution during the redox reaction of PBV. From cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) results, the exchange was almost complete around 50 cycles when scanning the potential within its first redox range. After completion of the exchange process, the transfer would reach a steady state. At 50 cycles, the EQCM results suggested that the transport involves anions and water only for both being extracted upon reduction and being inserted upon oxidation. Therefore, we could obtain the molar fluxes of Cl−, NO3−, and water. Besides, the average numbers of accompanying water were calculated to be about 24.8 per Cl− and 14.2 per NO3− upon redox switching process. The instantaneous water to anion molar ratios at any potential were also obtained for Cl− and NO3−.