Stable Elastomeric Anion Exchange Membranes Based on Quaternary Ammonium-Tethered Polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene Triblock Copolymers

A chemically stable and elastomeric triblock copolymer, polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS), was functionalized with various benzyl- and alkyl-substituted quaternary ammonium (QA) groups for anion exchange membrane (ARM) fuel cell applications. Synthetic methods involving transition metal-catalyzed C-H borylation and Suzuki coupling were utilized to incorporate six different QA structures to the polystyrene units of SEBS. Changes in AEM properties as a result of different QA moieties and chemical stability under alkaline conditions were investigated. Anion exchange polymers bearing the trimethylammonium pendants, the smallest QA cation moiety, exhibited the most significant changes in water uptake and block copolymer domain spacing to offer the best ion transport properties. It was demonstrated that incorporating stable cation structures to a polymer backbone comprising solely C-H and C-C bonds resulted in AEM materials with improved long-term alkaline stability. After 4 weeks in 1 M NaOH at 60 and 80 degrees C, SEBS-QA AEMs remained chemically stable. Fuel cell tests using benzyltrimethylammonium-containing SEBS (SEBS-TMA) as an AEM demonstrated excellent performance, generating one of the best maximum power density values and lowest ohmic resistance with low Pt catalyst loaded electrode reported thus far. Both polymer backbone and cation functional group remained stable after 110 h lifetime test at 60 degrees C.