Substrate‐Dependent Molecular and Nanostructural Orientation of Nafion Thin Films

Substrate‐Dependent Molecular and Nanostructural Orientation of Nafion Thin Films

TitleSubstrate‐Dependent Molecular and Nanostructural Orientation of Nafion Thin Films
Publication TypeJournal Article
Year of Publication2019
AuthorsDouglas I Kushner, Ahmet Kusoglu, Nikolas J Podraza, Michael A Hickner
JournalAdvanced Functional Materials
Volume29
Issue37
Pagination1902699
Date Published07/2019
ISSN1616-301X
Abstract

The effects of film thickness and substrate composition on the ionomer structure in porous electrodes are critical in understanding pathways toward developing higher performance electrochemical devices, including fuel cells and batteries. Insights are gained into the molecular and nanostructural orientation dependence for thin Nafion films (12–300 nm thick) on gold, platinum, and SiO2 model substrates. Molecular orientation is determined from the birefringence measured using spectroscopic ellipsometry, while the nanostructural orientation of the ionic domains is measured using grazing‐incidence small‐angle 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 SiO2 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 toward 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.

DOI10.1002/adfm.v29.3710.1002/adfm.201902699
Short TitleAdv. Funct. Mater.
Refereed DesignationRefereed