Redox-Driven Ion Exchange of Poly(3,4,-ethylenedioxythiophene) Films in Deep Eutectic Solvents
Authors: Hillman Robert , Zaleski Christopher , Fullarton Claire , Ryder Karl 
Published: February 2020

Poly(3,4-ethylenedioxythiophene) (PEDOT) is one of a range of conducting polymers based on aromatic heterocyclic monomers, typified by thiophene, pyrrole and their derivatives. The substitution pattern in PEDOT has the dual advantages of shifting the potentials for polymerization and redox switching into a more convenient range (as compared to the parent thiophene) and of blocking attack at the 3- and 4- positions of the aromatic ring. Both of these attributes enhance its practical application for energy storage applications, such as batteries and supercapacitors.

Quite generally, the rates of doping and undoping (charging and discharging, in the context of energy storage) of conducting polymers are limited not by the rate of electron transport along the polymer backbone, but by the transport rate(s) of the charge balancing ion(s), i.e. dopant(s). The dynamics of these processes have been widely studied during redox switching of PEDOT films immersed in diverse electrolyte media based on molecular solvent systems. Although the quantitative details vary with the electrolyte selected, the qualitative outcome is that anion transfer is commonly the dominant process accompanying PEDOT oxidation (p-doping), but that cation transfer (in the opposite direction) can be appreciable, and in any case solvent transfer is significant; in the context of nanogravimetric measurements (using the EQCM), the ion and neutral molecule transfers can be distinguished according to their correlation with transferred charge. Measurements of the absolute solvent population (cf. changes measured in an EQCM experiment) in many conducting polymer films reveal that this may represent ca. 50% by volume of the film: this facilitates both ion transfers and polymer spinal motions.

While such studies have revealed many interesting phenomena, notably solvation-controlled viscoelastic phenomena, a number of limitations make conventional solvent media unsuitable for practical applications; these may include available potential window, safety, toxicity or flammability. This motivates research on the redox chemistry of electroactive polymers in novel media, such as room temperature ionic liquids. Amongst these, we have been exploring the properties of deep eutectic solvent (DES) media based on various formulations of quaternary ammonium salts (QAS), hydrogen bond donors (HBD) and metal salts, in which complexation chemistry amongst the components generates fully ionic media. From a practical perspective, this provides an essentially unlimited source of ions (dopant). From a fundamental perspective, the dramatic difference is that these fully ionic media contain no solvent to plasticise the polymer; the more subtle difference is that the question of permselectivity that prevails (or fails) at low (or high) electrolyte concentration in a conventional solvent-based medium now becomes redundant.

Here we describe nanogravimetric acoustic wave (EQCM) studies of redox driven ion transfers for PEDOT films exposed to three DES formulations variously involving choline chloride (Ch+Cl-) as the QAS, ethylene glycol (EG) as the HBD and ZnCl2 as the metal salt. Viscosity effects associated with the DES are significant but, via acoustic admittance measurements, one can obtain film ion population change data. These are presented as a function of timescale (potential scan rate in a voltammetric experiment) for potentiostatically- and potentiodynamically-grown PEDOT films subsequently exposed to ZnCl2/EG, ZnCl2/acetamide and ZnCl2/Ethaline (where Ethaline is a 1:2 stoichiometric mixture of Ch+Cl- and EG). Significantly, for thicker films (consistent with practical applications), there is incomplete charge recovery at faster scan rates, with accompanying failure to restore the initial film ion composition. In multi-cycling experiments this leads to evolution of response; we discuss re-equilibration when the film is subsequently held in a resting state. Cycling over prolonged time intervals also results in evolution of film responses. The fundamental interpretation and practical consequences of these observations will be discussed.

We thank EU FP7 for financial support (NMP3-SL-2008-226655)