Stable Elastomeric Anion Exchange Membranes Based\non Quaternary Ammonium-Tethered Polystyrene‑<i>b</i>‑poly(ethylene-<i>co</i>-butylene)‑<i>b</i>‑polystyrene Triblock Copolymers

Abstract

A chemically\nstable and elastomeric triblock copolymer, polystyrene-<i>b</i>-poly­(ethylene-<i>co</i>-butylene)-<i>b</i>-polystyrene\n(SEBS), was functionalized with various benzyl- and\nalkyl-substituted quaternary ammonium (QA) groups for anion exchange\nmembrane (AEM) fuel cell applications. Synthetic methods involving\ntransition metal-catalyzed C–H borylation and Suzuki coupling\nwere utilized to incorporate six different QA structures to the polystyrene\nunits of SEBS. Changes in AEM properties as a result of different\nQA moieties and chemical stability under alkaline conditions were\ninvestigated. Anion exchange polymers bearing the trimethylammonium\npendants, the smallest QA cation moiety, exhibited the most significant\nchanges in water uptake and block copolymer domain spacing to offer\nthe best ion transport properties. It was demonstrated that incorporating\nstable cation structures to a polymer backbone comprising solely C–H\nand C–C bonds resulted in AEM materials with improved long-term\nalkaline stability. After 4 weeks in 1 M NaOH at 60 and 80 °C,\nSEBS-QA AEMs remained chemically stable. Fuel cell tests using benzyltrimethylammonium-containing\nSEBS (SEBS-TMA) as an AEM demonstrated excellent performance, generating\none of the best maximum power density values and lowest ohmic resistance\nwith low Pt catalyst loaded electrode reported thus far. Both polymer\nbackbone and cation functional group remained stable after 110 h lifetime\ntest at 60 °C.