A Donnan model was established to semi-quantify the trend of passive ion uptake in different polyelectrolyte (PSS -) content (see SI: Gibbs-Donnan Model in PEDOT:PSS). Shown in Figure 4d, the whole system was simplified as two phases: aqueous electrolyte with only mobile anion/cations and OMIECs with extra fixed charges (holes and PSS -) on the crystalline fiber. Ion concentration inside the OMIEC can be described by the following Donnan-Gibbs equation: