Anion Exchange Membranes Based on Polystyrene-Block-Poly(ethylene-ran-butylene)-Block-Polystyrene Triblock Copolymers: Cation Stability and Fuel Cell Performance

A series of polystyrene- block -poly(ethylene- ran -butylene)- block -polystyrene (SEBS)-based anion exchange membranes (AEMs) were synthesized via chloromethylation of SEBS followed by quaternization with trimethylamine (TMA). SEBS-based AEMs functionalized with TMA + cations exhibited a hydroxide ion conductivity of 136 mS/cm at 70°C. This membrane exhibited a phase-separated morphology with wide and interconnected ionic channels as observed by atomic force microscopy. Poly (phenylene oxide) (PPO)-based AEMs with quaternary benzyl-trimethylammonium (TMA + ) and quaternary benzyl-tris(2,4,6-trimethoxyphenyl) phosphonium (TTMPP + ) and PPO-based AEMs with hexyl pendant chains were also synthesized and evaluated as binders in AMFC electrodes. PPO-based AEMs functionalized with TTMPP + demonstrated the best ex situ alkaline stability, losing only 9% of their ion-exchange-capacity after 30 days in 1M KOH (at 60°C). The best in situ stability was achieved by SEBS-based MEAs (as membrane and as binder in the electrodes); The peak power density dropped approx. 35% after holding the cell at a constant voltage of 0.55V for 12 hours. Fuel cell performance with SEBS-based AEMs resulted in peak power densities of 300 mW/cm 2 at 70°C. The optimum performance was obtained with MEAs made with SEBS-based AEMs and PPO-based ionomers containing a hexyl pendant chain.