Ex situ XRF revealed that when exposed to RbBr electrolyte, EG/GOPS-PEDOT:PSS films underwent a large proton-cation exchange, with Rb + supplanting H + within the film. The equilibrium between proton and cation counterbalancing excess sulfonates was weakly dependent on electrolyte concentration and pH, showing significant differences only at the pH extremes and low metal halide concentration (Figure ). Thus, PSS -functioned as a buffer, minimizing the effect of electrolyte concentration and pH on OMIEC composition, except for high pH (11) conditions and the combined low pH (3) low salt concentration (1 mM RbBr) condition. In the initially exposed low pH (3) and low salt concentration (1 mM RbBr) conditions, where electrolyte proton and Rb + concentrations are equivalent, there is likely equal parts Rb + and protons within the film. For neutral electrolyte solutions near biologically relevant salt concentrations (100 mM), the Rb + concentration within the film effectively supplants the entire disassociated proton concentration. Initial cycles for some pH and electrolyte concentration conditions presented smaller than expected modulation of the cation concentrations, indicating a significant role of proton transport in initial film doping and dedoping. However, repetitive cycling appeared to break in the films and drive them towards a reversible equilibrium with the electrolyte that was dominated by metal cation transport. These multiple cycling experiments revealed a stabilized ~10% difference in Rb + concentration between the doped (0 V vs Ag/AgCl) and dedoped (-0.6 V vs Ag/AgCl) states (Figure ) across different pHs. While -0.6 V vs Ag/AgCl is not rigorously dedoped (zero electronic charge carrier density), it is sufficient to recover the majority of the neutral PEDOT species and arrive at the dedoped microstructure. Potentials necessary to drive PEDOT:PSS to the fully neutral state result in significant faradaic currents due to electrolyte breakdown or polymer degradation, and thus were avoided. When cycling from 0 to -0.6 V vs Ag/AgCl, for the neutral 100 mM RbBr condition, this represents a modulation of ~0.2 Rb + per ethylenedioxythiophene (EDOT) repeat unit. Given the presumed total doped hole density of ~0.33, this indicated much of the modulation in hole density is achieved through modulation of Rb + dopant density, though some minority proton contribution cannot be ruled out. The cation composition is reversible during multiple cycles of charging and discharging (Figure ). The Rb/S ratio increased slightly to 0.55 after multiple cycles in the dedoped state and 0.49 in the doped state.