Mechanisms of Diffusive Charge Transport in Redox-Active Polymer Solutions

Redox-active polymers (RAPs) have pendant groups that can change their charge state due to an electrochemical driving force. There has been an interest in using RAPs as a charge storage medium in redox flow batteries due to their ability to take on charge combined with their large macromolecular size. The performance of these batteries is in part tied to the transport of charge within these RAP solutions, and consequently there has been a recent effort to understand the physics governing charge diffusion in RAP systems. These efforts have highlighted the key role of both intra- and intermolecular charge transport mechanisms in governing their electrochemical response; however, little is known about how the molecular structure of polyelectrolyte solutions affects these proposed mechanisms. In this paper, we develop a coarse-grained, hybrid Brownian dynamics and kinetic Monte Carlo simulation to study charge transport in RAP solutions. We show how a number of different transport mechanisms interplay, including the intrapolymer transport of charge both along the chain via self-exchange transport and polymer segmental motions as well as hopping due to interpolymer collisions and translational diffusion of the chains themselves. We provide theoretical arguments to describe the diffusive motion of charge via these mechanisms, which match well with simulation results. Our predictions suggest the existence of three distinct regimes of charge transport, which distinguish between inter- and intramolecular processes and dilute and semidilute solutions.