Linking Density and Nanoscale Crystallinity to Hydration in Nafion PEMFC Membranes: Insights From Experiment and Molecular Dynamics Simulations

Proton exchange membrane fuel cells (PEMFCs) powered by hydrogen are promising for a wide range of energy‐conversion applications, with Nafion remaining the most widely used membrane material. Here, by comparing experimental data and molecular dynamics (MD) simulation results, we examine how a measurable property, the density, relates to the structure of Nafion at different water contents ( λ ). Voronoi tessellation and free‐volume analysis of MD results confirm that crystallinity and effective density are strongly correlated, highlighting the role of structural ordering. Further analysis of water clustering reveals isolated molecules at low hydration, while percolating networks form at higher λ , particularly within crystalline systems. Simultaneously, the density of confined water exhibits variations reaching values up to ≈1.2 g/cm 3 . We propose a model that bridges experimental and molecular observations by quantitatively linking water uptake to density variations, offering a predictive framework for optimising hydration and transport in membranes. Understanding how membranes with varying levels of crystallinity respond to changes in water content helps translate nanoscale mechanisms into practical designs for industrial PEMFCs.