Structures of polymer electrolyte membranes and binders and the distribution of water inside are important for designing new ion-conductive ionomers for polymer electrolyte fuel cells. Neutron reflectometry (NR) was carried out on a Nafion® film with a thickness of 100 nm formed on native SiO2 surface on Si(100) for understanding the in-plane water distribution. The temperature was set at 80 °C and the relative humidity at 30, 50, 65, and 80% for NR measurements, simulating the conditions for the power generation. Clear NR modulation was obtained under each condition. NR data were fit very well with a 4-layer model parallel to the substrate with different densities of Nafion and water. At the interface between the Nafion film and the Si substrate, a 1 nm water-rich layer was observed under all conditions. The water concentration increased with humidity at all 4 layers, but the thickness increased mainly at the bulk layer.
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