Separating the Double Layer from the Faradaic Processes at Conducting Polymer/Electrolyte Interfaces Using Electrochemical and Color Impedance Spectroscopies

The hybrid electrical-ionic transport properties of π-conjugated polymers offer a number of promising energy conversion and storage and biosensing applications for soft material semiconductors. Current studies focus predominantly on either electronic transport or ionic transport characteristics and structure-property relationships. As the electronic and ionic transports are highly coupled in electrochemical devices, it is a challenge to differentiate a redox process of the conjugated backbone (Faradaic) from the complementary intercalation of supporting electrolyte (non-Faradaic) at polymer/electrolyte interfaces with both energy and frequency resolution.<br/>Spectroelectrochemical approaches offer enhanced sensitivity to particular reactions of interest; in particular, polaronic motion can be monitored at various time scales upon doping using the electrochromic properties of π-conjugated systems. Herein we use color impedance spectroscopy (CIS) to resolve the dynamic responses of a prototypical system, poly(3-hexylthiophene), thus separating out contributions from non-polaronic processes – specifically ion intercalation and solvation effects – from Faradaic processes.<br/>Using CIS, we observe that higher doping potentials show a greater motion of polarons above the DC-bias baseline concentration, while all potentials considered demonstrates a critical frequency at which polaronic motion is frozen. This critical frequency offers a unique figure of merit, measured independent of complicated electrochemical impedance spectroscopy (EIS) analysis, by which to compare across polymer/electrolyte interfaces, including the role of charge-supporting electrolyte, solvent, and alternative Faradaic processes (ex. electrocatalysis).