Phase Transition-Governed Asymmetry in Poly(3-alkylthiophenes) Redox Kinetics: An Electrochemical Study

Poly(3-alkylthiophenes) are widely studied conducting polymers with charge transport occurring through both crystalline and amorphous regions. While their crystalline domains undergo structural changes during charge/discharge cycles, the impact of these transformations on redox kinetics remains unclear. Key questions persist regarding the rate-limiting steps of oxidation and reduction, the origins of their memory effect, and the inherent asymmetry between charge and discharge. In this work, we investigate the electrochemical behavior of poly(3-methylthiophene), poly(3-hexylthiophene), poly(3-octylthiophene), and poly(3-dodecylthiophene) to elucidate the role of alkyl group length in redox dynamics. Using chronoamperometry, cyclic voltammetry, impedance spectroscopy, quartz crystal microbalance, and in situ conductivity measurements, we quantify charge transfer-, diffusion-, and nucleation-related limitations. Our results reveal that oxidation is governed by an electrochemically driven phase transition with the limiting step being the nucleation rate. In the course of oxidation, anion intercalation into the closed (reduced) film is accompanied by the polymer opening process as the key limiting factor. Samples with larger alkyl groups require higher oxidative potentials to initiate the process of anion intercalation, which suggests the critical role of polymer structural opening via alkyl group reorganization. In contrast, anion deintercalation is fast and is not necessarily accompanied by the polymer closing process. Thus, reduction follows a single-phase pathway, showing thermodynamic control at low sweep rates and slow counterion deintercalation at high sweep rates. This asymmetry between charge and discharge processes leads to unusual electrochemical responses of such polymers, including asymmetric voltammetric peak currents, asymmetric shifts in peak potentials, and persistent hysteresis. These findings provide new insights into the electrochemical properties of polythiophenes and offer guidelines for optimizing their use as conductive additives and binders in energy storage applications.