Through-plane Structural Analysis of Engineered Nafion Surfaces*

The thin-film structures of Nafion, a model perfluorinated ionomer, impact practical advances in proton-exchange membrane fuel cells (PEMFCs). Engineered Nafion surfaces were developed by Dowd et al. to reversibly and permanently alter the surface composition and wettability of Nafion^[a]. Using neutron reflectometry (NR), we probe the through-plane structure of surface-treated Nafion thin-films with sub-Ångstrom resolution. Humidity-related layers of phase-separated lamellae were observed in Nafion thin-films at Au, Pt, and SiO₂ interfaces by Dura et al. and DeCaluwe et al^[b,c]. We develop and apply titanium nitride (TiN) substrates to minimize the neutron scattering contrast to similar Nafion-TiN interface structures, thus optimizing the sensitivity of NR to the thickness, roughness, and scattering-length density (SLD) of the engineered Nafion surfaces. The engineered Nafion thin-films are assessed in dry (0% RH) and wet (92% RH) conditions in H₂O and D₂O vapor to independently determine water content and polymer density, before and after surface modification. Additionally, we evaluate the formation structures at the Nafion-TiN interface.

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