Controlling the Kinetics of Charge Transfer at Conductive Polymer/Liquid Interfaces through Microstructure

Controlling interfacial electron-transfer rates is fundamental to maximizing device efficiencies in electrochemical technologies including redox-flow batteries, chemical sensors, bioelectronics, and photo-electrochemical devices. Conductive polymer electrodes offer the possibility to control redox properties through synthesis and processing, if critical structure-property relationships governing charge transfer are understood. In this work, we show that the rate and symmetry of electron transfer at conductive polymer electrodes are directly connected to the microstructure and the density of states (DOS) using the model system of poly(3-hexylthiophene) (P3HT) and ferrocene/ferrocenium (Fc/Fc⁺), as predicted by the Marcus-Gerischer model. Experimentally, crystalline P3HT exhibits a sufficient overlap between the polymer DOS and the DOS of both Fc and Fc⁺, resulting in a reversible electron transfer. Conversely, the DOS of amorphous electrodeposited P3HT does not overlap with that of Fc⁺, inhibiting reduction (i.e., kinetic selectivity for oxidation). This proof-of-concept work offers a paradigm to predict and control the kinetics at the polymer/liquid interface for applications from biology to energy.