Substrate‐Dependent Molecular and Nanostructural Orientation of Nafion Thin Films

Abstract

The effects of film thickness and substrate composition on the ionomer structure in porous electrodes is critical in understanding pathways towards developing higher performance electrochemical devices, including fuel cells and batteries. Insights are gained into the molecular and nanostructural orientation dependence for thin Nafion films (12 to 300 nm thick) on gold, platinum, and SiO 2 model substrates. Molecular orientation is determined from the birefringence measured using spectroscopic ellipsometry, while the nanostructural orientation of the ionic domains was measured using grazing-incidence smallangle X-ray scattering. Density functional theory calculations for the molecular polarizability of the Nafion backbone and side chain show complimentary contributions to the measured birefringence values for the material. Nafion films prepared on SiO 2 substrates exhibit a nearly isotropic molecular and nanostructural orientation. Films on gold and platinum display parallel backbone orientations, relative to the substrate, with decreasing film thickness. However, a birefringence transition towards molecular isotropy is observed for 30 nm thick films on Au and Pt; while the ionic nanostructured domains continuously align parallel to the substrate. This apparent isotropic molecular orientation with increasing domain orientation highlights the difference between the backbone and side chain orientation, a key finding for elucidating transport in confined films at the interfaces.

Progress on highly proton-conductive polymer thin films with organized structure and molecularly oriented structure

“…The oriented structure of Nafion thin films depends on the thickness of sputtered Pt and Au surfaces [36,37], although this large structural change surfaces were oriented to the in-plane direction to the substrate surface and that the main chain orientation was enhanced with decreasing thickness. This discussion of main-chain orientation can be supported by the work of another research group [64]. Figure 2(g-j) presents the film thickness dependence of the in-plane proton conductivity on quartz [37], MgO [35,36], sputtered Pt [37], and sputtered Au [36] surfaces.…”

Anion Exchange Ionomers: Impact of Chemistry on Thin‐Film Properties

“…Likely, these features could also be affected by substrate type (i.e., metallic versus dielectric); however, it is unclear how the presence of the quaternary ammonium will affect interactions with metallic substrates as has been shown with sulfonated polymers on metallic substrates. [ 19 ]…”

SAXS signature of the lamellar ordering of ionic domains of perfluorinated sulfonic-acid ionomers by electric and magnetic field-assisted casting

“…5e. The lamellar ordering of ionic aggregates has already been postulated for Nafion membranes [6,7,9,10,11]. Two distinct lamella stacking possibilities were previously proposed for Nafion [16,48].…”

“…On the other hand, it is predicted that the electrostatic interactions existing within locally flat or lamellar ionic phase are stabilized by thin disk-like water domains that act as a "glue" holding the nanostructure together [6,7]. In fact, in several reports, the lamellar arrangement of the ionic domains in Nafion has been inferred or at least suggested [6,7,9,10,11]. In this scenario, the SAXS modelling of PFSAs lacks evidence-based findings to be used as pivot data for the convergence of a more plausible mathematical/phenomenological model of Nafion.…”