In-Plane Distribution of Water inside Nafion Thin Film Analyzed By Neutron Reflectivity at Temperature of 80 °C and Relative Humidity of 30-80%

Understanding the interfaces in an MEA of a PEFC, or the reaction fields, has become very important, and their structures and properties are intensively studied. Bulk and interfacial structures of polymer electrolyte membranes and binders, as well as the distribution of water inside, are important for designing new ion-conductive ionomers for polymer electrolyte fuel cells. Neutron reflectivity (NR) is a powerful method to observe structures of electrolyte polymers on flat substrates with an Angstrom precision in the in-plane direction. NR measurements were carried out on model Nafion films with a thickness of approximately 100 nm formed on a native SiO2 surface and on a Pt layer on Si(100) for understanding the in-plane Nafion and water densities.1) Under N2 humidified with H2O, the temperature was set at 80 ˚C and the relative humidity between 30% and 80% during the NR measurements, simulating the conditions for the power generation. Figure 1 shows the NR profiles of Nafion at 80 °C at different relative humidities on SiO2 and Pt, respectively. Clear NR modulation was obtained under each condition on each specimen. The NR profiles on SiO2 and Pt were very different, showing different film morphologies. The characteristic bump around 0.2 A-1 observed for Nafion on SiO2 (Fig. 1(a)) could be explained by the existence of a layer with a very different scattering length density (SLD) from that of Nafion, which was not observed for Nafion on Pt (Fig. 1(b)). The analyses of the NR profiles were carried out using the MOTOFIT program. For Nafion on SiO2, the NR data in Fig. 1(a) were fit very well with calculated values applying 4-layer models. Figure 2 shows the layered model of Nafion on SiO2/Si(100). SLD values were converted into Nafion and water densities at each sub-layer. The total thickness of the Nafion film at 80 oC was 131, 139, 146, and 153 nm at 30, 50, 65, and 80% RH, respectively. The water uptake calculated from the NR data at 80% RH of a 100-nm Nafion on SiO2 was in good agreement with that measured by the mass of a film at 80 oC.2) In "Topmost Layer" in Fig. 2, the water density was very low, and a hydrophobic conformation at the interface between Nafion and humidified N2 was proposed. The thickness of "Bulk Layer" monotonically increased from 120 to 140 nm as the humidity increased from 30 to 80% RH. The film thickness of "Intermediate Layer" slightly increased from 2.3 to 3.0 nm. The water density in "Intermediate Layer" was two times higher than that in "Bulk Layer". The thickness of "Water-rich Layer" was 1.3 nm regardless of the humidity, while the water density greatly increased up to 0.97 g cm-3 at 80% RH. As seen in Fig. 1(b), no bump was observed on Nafion on Pt. The Nafion film on Pt/Si(100) was modeled with three layers with no "Water-rich Layer". The influences of different substrates are clearly observed, which will be discussed in detail. This work was performed under the SPer-FC project of NEDO, Japan. The NR experiments were performed as projects approved by the Japan Proton Accelerator Research Complex under user programs, Nos. 2016A0246, 2016B0036, and 2017B0316. References 1) T. Kawamoto, M. Aoki, T. Kimura, T. Mizusawa, N. L. Yamada, J. Miyake, K. Miyatake, and J. Inukai, accepted to Jpn. J. Appl. Phys. 2) C. Yang, S. Srinivasan, A. B. Bocarsly, S. Tulyani, and J.B. Benziger, J. Membr. Sci. 237, 145, (2004). Figure 1