Low water swelling polyaromatic proton exchange membranes

Abstract

A carbon-carbon linked polyaromatic electrolyte was designed and synthesized for proton exchange membranes. The facile synthesis consists of a metal-free Friedel-Crafts polymerization and a direct post-sulfonation. To prevent the random sulfonation, the trifluorobuanone monomer was incorporated as a spacer to sterically confine the sulfonated area. Hydrophobic/hydrophilic separation was observed in the polymer's microstructure. The membrane (ion exchange capacity = 1.88 mmolg−1) displayed low water swelling (area swelling = 38.0% at 80 °C) at a slightly higher proton conductivity than Nafion 212. Tensile strength of 39.3 MPa and elongation at break of 67.0% were confirmed by stress-strain measurements. The membrane electrode assembly afforded an electrolyzing performance of 4.6 A cm−2@2.0 V while Nafion 212 was 3.6 A cm−2@2.0 V under the same testing conditions. The 45 μm film exhibited better stability than Nafion 212 (∼50 μm) both in the preliminary in-situ cyclic accelerated stress test and the 800-h galvanostatic evaluation. The degradation rate of our thin membrane in the 800-h test was comparable to that of Nafion 115 (∼125 μm).