Characterization of Thin Film Nafion Structure via Neutron Reflectometry and Complementary Techniques

The thin film structures of Nafion, the most widely used proton-exchange ionomer, impact the performance of proton-exchange membrane fuel cells (PEMFCs). Engineered Nafion surfaces were developed by Dowd et al. to alter the surface composition and wettability of Nafion to be hydrophobic or hydrophilic^[a]. Using neutron reflectometry (NR), we probe the through-plane structure of Nafion thin films to characterize potential structures at the engineered Nafion surfaces. Buried layers of phase-separated water/Nafion lamellae were observed in Nafion thin films at Au, Pt, and SiO₂ interfaces by Dura et al.^[b] and DeCaluwe et al^[c]. For this work, a titanium dioxide (TiO₂) substrate was applied to minimize the neutron scattering contrast to potential interface structures and enhance the NR sensitivity to the surface structure of engineered Nafion.

NR data of the neat Nafion:TiO₂ sample taken in dry (Ar, 0% RH) and humidified (H₂O and D₂O, 92% RH) environments are compared to determine the Nafion water content as a function of depth through the film. A non-lamellar, mixed-SLD region at the Nafion:TiO₂ interface was observed.

NR results of the hydrophobic and hydrophilic-engineered Nafion thin films on TiO₂ are compared to the neat films to determine water uptake differences and changes to the Nafion surface structure following modification.

^[a]10.1149/2.1081702jes ^[b]10.1021/ma802823j ^[c]10.1039/C4SM00850B