Ion transfer dynamics of poly(3,4-ethylenedioxythiophene) films in deep eutectic solvents

Mechanistic studies are reported for redox switching (doping/undoping) of Au-supported poly(3,4-ethylenedioxythiophene) (PEDOT) films exposed to LiClO4/CH3CN (a conventional electrolyte based on a molecular solvent) and to Ethaline and Propaline (two choline chloride-based deep eutectic solvent (DES) media). A combination of electrochemical, acoustic (quartz crystal microbalance, QCM) and optical (probe beam deflection, PBD) methods was used to monitor the exchange of mobile species between the film and the bathing electrolyte. Qualitatively, film responses to a potentiodynamic control function showed that the redox switching mechanisms are quite different in all three media. When exposed to acetonitrile, anion transfer is dominant, with some accompanying solvent transfer but negligible cation transfer; application of the convolution protocol allowed these transfers to be quantified. Analogous observations in the DES media could be interpreted qualitatively in terms of dominant cation transfer, but the convolution protocol could not be used to quantify the contributions of individual species; in these viscous media, this is a consequence of the long transit times from the film/solution interface to the optical detection zone. Chronoamperometric experiments, in which the measurement time was an order of magnitude (or more) longer permitted the diffusional processes driven by the polymer/DES interfacial population changes to reach the optical detection zone.