Side Chain Engineering of Proton Exchange Membranes with Tunable Acidity

Proton exchange membranes (PEM) are a crucial material component for clean and sustainable energy conversion technologies including fuel cells, electrolyzers, and redox flow batteries. The current state-of-the-art PEMs rely on perfluorinated sulfonic acid (PFSA) polymers, which possess several inherent drawbacks including limited synthetic flexibility, high cost, and environmental concerns. For this reason, among others, hydrocarbon PEMs have gained attention recently. In this work, we disclose a synthetic method of hydrocarbon PEMs starting from anion exchange membrane precursor polymers that affords increased acidity of sulfonate group without incorporating a PFSA unit on the side chain. The unique polymer design here utilizes site-selective post-polymerization sulfonation to obtain well-defined PEMs, BP-SArSA and BP-SOArSA, using simple and convenient synthetic routes. These PEMs showed excellent proton conductivity in liquid water with minimal water uptake and swelling, good oxidative stability in the ex situ Fenton’s reagent test, and good electrolyzer cell performance comparable to Nafion 212.