Morphological Differences in Short Side Chain and Long Side Chain Perfluorosulfonic Acid Proton Exchange Membranes at Low and High Water Contents

Abstract

Ionomer membrane materials such as Dupont’s Nafion® (a sulfonated tetrafluoroethylene based fluoropolymer) have long been used as the proton conducting medium in proton exchange membrane (PEM) fuel cells. It has recently been shown that short side chain perfluorosulfonic acid membranes such as Hyflon Ion® developed by Solexis can outperform the benchmark long side chain material, DuPont’s Nafion®. Despite these promising results, a detailed understanding of the relationship between membrane morphology, hydration, and fuel cell performance has yet to be achieved. Using atomic force microscopy (AFM) phase imaging over a wide range of relative humidity and in solution, we compare a Nafion 212 membrane to Hyflon Ion® E87-05 and find evidence of the improved performance in the latter. Hyflon exhibits a higher hydrophilic surface area across all water contents, explaining its higher proton conductivity and fuel cell performance. At high water contents, the surface of Nafion® undergoes significant structural rearrangement and forms large rod-like features parallel to the surface, whereas the Hyflon Ion® membrane undergoes little if any such transition. [see the figure] We show using conductive AFM that these features are insulating, which indicates they most likely consist of a fluorocarbon layer surrounding a hydrophilic core and do not contribute to fuel cell performance. The ability of Hyflon to undergo changes in water content with minimal rearrangement of hydrophilic domains implies an ability to operate more effectively under variable relative humidity.