Understanding Electrochemical Doping in Carboxylated Mixed Ionic-Electronic Conductors: From Ion Uptake to Functional Performance

Abstract

Organic mixed ionic-electronic conductors (OMIECs) are emerging as key materials for energy storage, bioelectronics, and sensing due to their dual ionic and electronic conductivity. However, the complex interplay between chemical structure, environment, and ion dynamics remains poorly understood. This thesis advances the understanding of electrochemical doping mechanisms in carboxylated polythiophenes by probing ion uptake, transport, and swelling behavior in real time. Chapter 2 investigates the influence of COOH functionality and alkyl spacer length on polymer performance, revealing materials that exhibit low swelling, high aqueous processability, and strong ionic-electronic coupling. Benchmarking studies demonstrated record-setting OECT performance, supported by in situ spectroelectrochemical analysis. Chapter 3 explores the impact of side-chain chemistry by comparing carboxylic acid and ester-functionalized analogs. Operando characterization showed that COOH groups facilitate cation expulsion and deswelling during doping, while ester groups enable cation-free, anion-driven swelling—underscoring the critical role of side-chain polarity in ion transport dynamics. Chapter 4 focuses on pH-regulated doping behavior. The degree of COOH dissociation, governed by its pKa, was shown to modulate both ion uptake and swelling: neutral pH promotes deswelling through stronger cation-polymer interactions, while acidic pH limits dissociation and leads to increased swelling. This work introduces the concept of an electrochemical dissociative balance, where ion flux and volume changes can be tuned without requiring swelling. Collectively, these findings establish key design principles for carboxylated OMIECs with enhanced performance, long-term stability, and minimized swelling across diverse electrochemical environments.</p