Effect of Flow on Charge Transport in Semidilute Redox-Active Polymer Solutions.

Abstract

Redox-active polymers (RAPs) are polyelectrolytes that can undergo redox self-exchange reactions and can thus store charge. This ability makes them of interest as an electrolyte material in redox flow batteries due to their molecular size, chemical modularity, and ability to quickly charge and discharge. It is therefore important to understand how charge is transported at the molecular level, under different conditions such as RAP concentration, flow type, and flow strength. While previous efforts have explored these mechanisms in detail, they have primarily focused on charge transport dynamics in equilibrium or in dilute nonequilibrium situations. In this work, we seek to build upon these previous models by accounting for both nonequilibrium dynamics into semidilute RAP solutions and showing ways in which intermolecular interactions couple to strong flows to affect charge transport. Using recent advances in modeling multichain systems in flow, we show that for a single charge, both extensional and shear flow promote charge transport by extending the polymer conformation. This allows the charge to hop along a longer path along the same chain while also increasing the number of chain-chain collisions needed for interchain hopping. We also show that, when multiple charges are present in equilibrium, the charge transport decreases monotonically at all polymer concentrations but decreases the most below the overlap concentration. We attribute this to a decreased probability of chain-to-chain collisions, leading to a concomitant decrease in charge hopping with increasing charge fraction. Overall, we show how charge fraction, concentration, and flow strength couple to produce enhanced charge transport in concentrated solutions in both extensional and shear flows at semidilute concentrations.