Morphology and Structure of PEDOT:Nafion: Insights from Experiment and Molecular Dynamics Simulations

Poly(3,4-ethylenedioxythiophene) (PEDOT)/Nafion is a promising mixed ionic–electronic conducting polymer. However, in contrast to the well-studied PEDOT:poly(styrenesulfonate) (PSS) system, the nanoscale morphology and the relationship between crystallinity and composition in PEDOT/Nafion remain insufficiently explored and therefore poorly understood. In this work, we investigate the morphology and structural organization of PEDOT/Nafion by combining X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and coarse-grained molecular dynamics(MD)simulations. This combined approach enables usto directly link surface composition, bulk crystallinity, and molecular-scale packing in PEDOT/Nafion—insights that are not accessible from PEDOT:PSS or Nafion-only studies. XPS revealed two distinct sulfur environments associated with PEDOT and Nafion, with a sulfonic-acid-to-thiophene-group ratio RS/T of ≈ 2.3. XRD analysis showed PEDOT crystallites with an average π–π stacking size of about 2.1 nm. MD simulations provided molecular-level insight into the effect of composition, demonstrating that higher Nafion content disrupts PEDOT stacking, yielding smaller crystallites (∼1.1 nm), while reduced Nafion content promotes better ordering (∼1.4–1.8 nm). These findings establish a clear correlation between composition and crystallinity, supporting a model where the surface is Nafion-rich and the bulk has a lower RS/T ratio. The combined experimental–computational approach offers a comprehensive understanding of PEDOT morphology and valuable guidance for the design of mixed ion–electron conducting polymers.