Deciphering Concentration and Backbone Identity Effects on the Redox Properties of Phthalimide-Based Polymers

Abstract Non-conjugated redox-active polymers (NC-RAPs) in the dissolved state are important for future energy applications such as redox flow batteries. However, there remains a knowledge gap in understanding factors that strongly influence the redox kinetics and mechanisms of NC-RAPs. Here, we determine how concentration and backbone structure of phthalimide-based NC-RAPs influence the solution-state electrochemical activity, as described by the apparent diffusion coefficient (Dapp) and the homogeneous (kex,app) and heterogeneous (k0) rate constants. Three phthalimide-containing polymers with epichlorohydrin, methacrylate, and vinylbenzene backbones are compared experimentally and computationally. As polymer concentration increases from the dilute to the semidilute regimes, the diffusion and kinetics slow down. Interestingly, the polymer backbone does not strongly influence the redox kinetics of these phthalimide-based polymers. In total, these results show that, above the overlap concentration, single-file diffusion and interdependent flux of redox species lead to slower diffusion and kinetics. These findings provide mechanistic insights into the future molecular design of NC-RAPs in the dissolved state, enabling energy storage systems such as redox flow batteries with improved kinetics.