Sublayered Thin Films of Polymer Electrolyte for Fuel Cells Formed on SiO2 and Pt Substrates Analyzed By Neutron Reflectometry Under Controlled Temperature and Humidity

Abstract

Three-dimensional structures of polymer electrolyte membranes and binders under controlled temperature and humidity, as well as the concentrations of water in the electrolytes, are important for designing ionomers for fuel cells. A large body of work exists on bulk electrolyte membrane and catalyst layer, but less is known about the structure and behavior of ionomer on the nanometer scale especially at the interfaces. Neutron reflectometry (NR) is known as a powerful technique to study layered structures, but the investigations of polymer electrolyte thin films have been conducted mainly at room temperature, and thin films of anion exchange ionomers have rarely been examined. Here, under controlled temperature and humidity, we investigated the distributions of water by NR inside thin films of Nafion and BAF-QAF (Fig. 1), proton and anion exchange ionomers, respectively.1-3 Thin films were prepared by spin-coating dispersions of Nafion/alcohol and BAF-QAF/N,N-dimethylacetamide (DMAc) on SiO2/Si(100) and Pt/SiO2/Si(100). The Nafion and BAF-QAF specimen films were placed in an environment-controlled chamber continuously purged with N2 gas humidified with H2O for Nafion and D2O for BAF-QAF. NR measurements were carried out at the beamline BL16 of MLF, J-PARC, Japan. Figure 2 shows the NR profiles of Nafion (I) and BAF-QAF (II) films on SiO2, and Nafion (III) and BAF-QAF (IV) films on Pt. The symbols show the reflectivity data, whereas the solid lines the best-fit curves. Clear and different NR modulations were observed for Nafion and BAF-QAF films on different substrates. Figure 3 shows the structure models of Nafion on SiO2 and on Pt at 80 ° C and 80% RH, proposed from the fitting of NR curves (Fig. 2). On the SiO2 substrate, a 4-sublayered structure model was proposed for the Nafion film (Fig. 3(a)). The “Topmost Layer” of the interface between Nafion and humidified N2, 9.1 nm thick, contained only 0.20 g cm-3 of H2O. A large portion of the Nafion film was occupied by the "Bulk Layer" with 0.24 g cm-3 of H2O. The thickness of the “Intermediate Layer” was found to be 3.0 nm with an increasing H2O density of 0.44 g cm-3. The 1.2-nm thick “SiO2-attached Layer” at the interface between Nafion and SiO2 showed an extremely high H2O density of 0.97 g cm-3. On the Pt substrate, a 3-sublayered model was proposed to the Nafion film (Fig. 3(b)). The H2O density of the “Topmost Layer” was 0.22 g cm-3. A large portion of the Nafion film on Pt was also occupied by the “Bulk Layer” with a small H2O density, 0.16 g cm-3. The thickness of the "Pt-attached Layer" was less than 1 nm, and the density of water was 0.60 g cm-3, smaller than that on SiO2. Figure 4 shows the 3-sublayered structure models of the BAF-QAF films on SiO2 and on Pt at 60 ° C and 90% RH. The “Topmost Layer” of BAF-QAF on SiO2 contained the largest amount of water among the four specimens examined, with a D2O density of 0.44 g cm-3 (Fig. 4(a)). The thickness of the "SiO2-attached Layer" was 1.1 nm, and the D2O density was 0.26 g cm-3, similar to that of the “Bulk Layer”, 0.24 g cm-3. On Pt, the “Topmost Layer” was found to be rather hydrophobic with 0.15 g cm-3 (Fig. 4(b)). The "Pt-attached Layer", 4.3 nm, had a very small amount of water with a D2O density of 0.09 g cm-3. The water densities, thereby the nanostructures, at the interfaces were very different from those of bulks; the mass and energy transports through the membranes and binders in membrane-electrode assemblies should be much influences by the interfaces. Among all films, the interface between BAF-QAF and Pt was the most hydrophobic; the water density (0.09 g cm-3) was only one sixth of that of Nafion (0.60 g cm-3). In the AEMFCs, the nucleophilicity and basicity of the hydroxide ion increased with the decreasing hydration number of the hydroxide ion, resulting in the rapid chemical degradation of the ammonium groups.4 The hydrophobicity of the anion binder/Pt interface might be a cause of the lower performance and the durability of AEMFCs.(1) T. Kawamoto et al. Jpn. J. Appl. Phys. 2019, 58, SIID01.(2) T. Kawamoto et al. Electrochemistry 2019, 87, 270-275.(3) T. Kimura et al. Langmuir 2020, 36, 4955−4963.(4) D. R. Dekel et al. Chem. Mater. 2017, 29, 4425–4431. Figure 1