<i>In Situ</i> Small-Angle X-ray Scattering Investigation of Hydrated Membrane Nanostructures in Polymer Electrolyte Fuel Cells

Understanding the hydration state of polymer electrolyte membranes (PEMs) in operating fuel cells is essential for enhancing performance and ensuring long-term durability. Here, we present an in situ small-angle X-ray scattering (SAXS) approach that enables direct observation of PEM hydration within membrane electrode assemblies. A dedicated in situ SAXS cell was developed to suppress scattering from catalyst layers and isolate membrane-specific signals. In situ measurements revealed that break-in procedures enhanced the structural ordering of the nanophase-separated water domains in the hydrated PEM without altering the water-domain spacing. Systematic measurements further showed that water-domain spacing increased at lower cell temperatures and higher current densities, providing direct structural evidence of hydration dynamics in PEMs. These findings demonstrate that in situ SAXS is a powerful tool for probing membrane nanostructures under realistic fuel cell conditions.