Sublayered Thin Films of Hydrated Anion Exchange Ionomer for Fuel Cells Formed on SiO 2 and Pt Substrates Analyzed by Neutron Reflectometry under Controlled Temperature and Humidity Conditions

Anion-conductive ionomers are used for electrolyte membranes in membrane-electrode assemblies and for binders in catalyst layers in anion exchange membrane fuel cells (AEMFCs). The conformations of these ionomers as well as their water distribution are important for designing new efficient/durable anion-conductive ionomers for AEMFCs. For a deeper understanding of the distribution of deuterium oxide (D O) as a function of depth, neutron reflectometry (NR) was carried out on thin films of an anion exchange ionomer, BAF-QAF, with a thickness of approximately 60 nm formed on a thermally formed SiO film on Si(100) and on a 20 nm Pt layer deposited on the SiO film at a temperature of 60 °C and relative humidities of 0, 50, 70, and 90%. Clear NR modulation was obtained under each condition. The NR data were fit very well with a three-sublayered model parallel to the substrate with different densities of BAF-QAF and D O. The influence of the SiO and Pt substrates was observed not only at the BAF-QAF/substrate interface but also on the entire thin film. The D O absorption/desorption behavior in each sublayer differed in the BAF-QAF films cast on SiO and Pt. The BAF-QAF/SiO interface was rather hydrophilic, while the BAF-QAF/Pt interface was very hydrophobic.