Ion transport across the film of poly(5,6-dimethoxyindole-2-carboxylic acid) in relation to its electrochromic switching: An electrochemical quartz crystal microbalance study

An electrochromic thin film of the conducting polymer, poly(5,6-dimethoxyindole-2-carboxylic acid) (PDMICA), was prepared by electropolymerization. The transport phenomena of anions, cations, and water molecules through the film of PDMICA were studied in the electrolytes of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), LiNO3, and NaNO3, by means of electrochemical quartz crystal microbalance (EQCM), intending to throw more light on its electrochromic behavior. Cations do not affect the ion transport, but anions play an important role during the reduction and oxidation of the film. Changes were observed in the molar fluxes of anions and water molecules in the PDMICA films in the systems of LiTFSI and LiNO3, depending on the applied potential. Besides, 14.7 and 3.4 water molecules per TFSI− and per NO3−, respectively, were estimated during the redox switching of PDMICA in LiTFSI and LiNO3. The anions and water molecules were found to dominate the electrochromism with the film of PDMICA.