Sublayered structures of hydrated Nafion^® thin film formed by casting on Pt substrate analyzed by X-ray absorption spectroscopy under ambient conditions and neutron reflectometry at temperature of 80°C and relative humidity of 30-80%

The structures of polymer electrolyte membranes and catalyst layer binders and the distribution of water therein are important for designing new ion-conductive ionomers for polymer electrolyte fuel cells. To aid the understanding of the in-plane water distribution, neutron reflectometry (NR) was carried out on a Nafion^® film with a thickness of 150 nm formed on a 20-nm Pt layer deposited on Si(100) with a native SiO₂ layer. By means of ambient pressure X-ray absorption spectroscopy at room temperature in air, the Pt substrate was found to be metallic. For NR, the temperature was set at 80°C and the relative humidity at 30, 50 and 80%, simulating the conditions for power generation. Clear NR modulation was obtained under each condition. NR data were fit very well with a 3-sublayered model parallel to the substrate with different densities of Nafion and water. The influence of the Pt substrate was observed not only at the Nafion/Pt interface, but also on the thin-film structure. The water uptake in a Nafion film on Pt also differed from that on SiO₂. At 80°C, the surface of the Pt substrate was proposed to be oxidized, and the Nafion/Pt interface was found to contain water, in contrast to the interface observed at room temperature.