Morphological differences in short side chain and long side chain perfluorosulfonic acid proton exchange membranes at low and high water contents

Abstract

Short side chain (SSC) perfluorosulfonic acid membranes such as Hyflon Ion® have shown improved proton conductivity and the ability to outperform the benchmark long side chain (LSC) material, Nafion®. Using atomic force microscopy phase imaging over a wide range of relative humidity, we compare the morphology of Nafion to Hyflon and find evidence for the improved performance in the latter. While the surface of Nafion undergoes significant structural rearrangement and forms large worm-like features at high water contents, Hyflon undergoes little, if any, transition. We also provide evidence of improved water retention in Hyflon at low water contents, explaining the improved conductivity under these conditions. Liquid phase imaging in water and methanol solutions was used to probe the extremes of swelling in both membranes and conductive AFM was used to characterize electrochemical activity at the membrane surface.