We report here the of dependence of the polymer ionic domain structure and water uptake on the thickness of ultrathin Nafion films (ranging from 5 – 153 nm) coated onto hydrophilic SiO2 substrates, using in situ neutron reflectometry (NR) as a probe. NR is capable of determining structure with sub-Angstrom precision in idealized planar interfaces. Because of isotopic variations in neutron scattering cross sections, it is particularly sensitive to certain elements such as H (and thus water) as well as Li, V, and numerous others. Our results herein show that ionic domains form as sheet-like lamellae at the SiO2 interface, with a thick, bulk-like layer forming at the lamellae/vapor interface for films thicker than 20 nm. The bulk-like layer water uptake increases with increasing thickness for films with equivalent Nafion thickness tNaf < 60 nm, but these bulk-like layers have constant water uptake, similar to bulk 1100 eq. wt. nafion (l = 10) for tNaf ≥ 60 nm. We also show that while the lamellae form due to substrate interactions, their structure and water uptake are also influenced by the presence and composition of the bulk-like layer. Finally, these results indicate that inferring water content from whole-sample measurements (such as swelling or mass uptake measurements) can, in some cases, give an insufficient or even inaccurate measure of the Nafion water uptake.
These structural measurements from NR are then used to predict the ionic conductivity of ultrathin Nafion films, using previously established relationships for sion based on l. Fitting these predictions to experimental data provides evidence of transport limitations in the Nafion film, which increase with increasing proximity to the substrate, and helps to explain previously counter-intuitive trends in ionic conductivity vs. thickness for these films. These effective conductivities are then implemented in numerical simulations of the PEMFC anode and cathode catalyst layers, and demonstrate that both finite thickness effects and sheet-like ionic domain structures at film interfaces can have significant implications for PEMFC performance.