Spectroelectrochemical Methods for Polaronic Motion in Energy Conversion and Storage

Abstract

The hybrid electronic–ionic transport property of π-conjugated polymers enables new (opto)electrochemical device constructs for energy conversion and storage and biosensing applications. The key operation mechanism for organic electrochemical devices relies on hybrid carrier transport that involves electronic transport, ionic transport and electronic-ionic couplings. The former two have been widely characterized by potential-dependent structure-to-property relationships while there is a lack of understanding on electronic-ionic couplings. The challenges arise from the difficulty to characterize electronic-ionic couplings under operation: how to differentiate a redox process of the conjugated backbone (Faradaic process of charge transfer involving polarons) from the complementary intercalation of the supporting electrolyte (non-Faradaic resulting from ionic transport); and how to probe the polaronic motion under coupling effect in local environments (i.e., crystalline domains, short-range order regions and amorphous domains) with certainty? Herein, we leverage the species specificity of spectroscopy combined with energy and frequency resolution of impedance spectroscopy to monitor polaronic motions in different polymers using color impedance spectroscopy (CIS). The movement of polarons can be distinguished in different local environments. This study highlights the complexity of the role of microstructure in polaronic transport under electronic-ionic coupling and provides new insights for future materials design in order to accommodate the different functionality of organic electrochemical devices. Future studies can emphasis on the role of the charge-supporting electrolyte, solvent, and alternative Faradaic processes (e.g., electrocatalysis) on electronic-ionic couplings.