Charge Transfer and Mass Transport in Organic Radical Polymers

Organic radical polymer batteries offer an intriguing path towards plastic energy storage. Organic radical polymers often consist of an aliphatic backbone and pendant stable radical groups. These electroactive polymers store charge by a reversible redox reaction, usually with near-100% Coulombic efficiency and extremely fast charge transfer kinetics. However, their performance is complicated by the generally insulating nature of the polymer backbone. Also, the general nature of this reaction is not completely understood, as it is fundamentally different from more traditional battery electrode materials and conjugated polymers. Here, we present electrochemical quartz crystal microbalance with dissipation (EQCMD) monitoring as a means to quantify ion transport, dynamic swelling, and mechanical properties of an organic radical polymer during electrochemical interrogation. We focus upon EQCMD characterization of the redox-active nitroxide radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA). Upon oxidation, PTMA becomes positively charged, which requires the transport of a complementary anion into the polymer for electroneutrality. By EQCMD, we quantify anion transport and resultant swelling upon oxidation, as well as decoupling of contributions attributed to the ion and the solvent. We explore the effect of different lithium electrolyte salts in which each salt gives different charge storage and mass transport behavior. This is attributed to varied polymer-dopant and dopant-solvent interactions.