Beyond Swelling: Electrochemical Doping Tunes the Balance between Ion Compensation and Exchange in a Hydrated Organic Mixed Ionic–Electronic Conductor

Abstract Organic mixed ionic–electronic conductors (OMIECs) couple electronic doping to ion compensation and water reorganization. As doping increasing, molecular reorganization within the polymer–ion–water environment is thought to underlie the often nonmonotonic response of OMIEC devices, yet this process is essential for the understanding of structure–function relationship in related OMIEC devices but remains unclear. Although advanced operando measurements can resolve charge-state-dependent counterion localization within ordered OMIEC domains, jointly tracking electronic charging, explicit ion compensation, hydration, and molecular exchange across doping remains challenging; conventional simulations likewise do not allow these coupled variables to evolve together under constant electrochemical potential. To address these limitations under experimentally relevant electrochemical conditions, we develop an ion-regulated quantum mechanics/molecular mechanics (QM/MM) grand-canonical molecular dynamics framework that operates at constant electrochemical potential and samples OMIECs across experimentally relevant doping levels. Experimental 23Na and 35Cl nuclear magnetic resonance (NMR) spectra and layer-resolved computational sum-frequency-generation (SFG) spectra validate the polymer microstructure and polymer–electrolyte interfacial model. The simulations reveal a doping-dependent reorganization mechanism and identify a distinct compensation–exchange balance at the charging level associated with the experimental carrier-mobility maximum. With increasing doping, water–side-chain hydrogen bonding weakens and the glycol-rich environment crosses over from Na+ organization to Cl–-dominated compensation. Near the mobility maximum, persistent Na+ environments and a long-lived Na+–Cl– motif coexist with a more rapidly exchanging Cl– population. Further charging disrupts this balance, producing either kinetically trapped Cl– or a more highly hydrated, rapidly exchanging environment. These findings establish distinct ion-compensation and exchange regimes across doping and provide a general framework for resolving polymer–ion–water structure–function relationships in OMIECs.