Polystyrenes containing flexible alkylsulfonated side chains as a proton exchange membrane for fuel cell application

Abstract

Polymer electrolyte membranes (PEMs) of cross-linked polystyrene derivatives, poly{(4-butoxystyrene)_x-r-[4-(4-sulfobutyloxy)styrene]_y} (PBOS_x-r-PSBOS_y), based on a hydrophobic alkoxystyrene unit and a hydrophilic styrene unit containing a pendant alkyl sulfonic acid with the ion exchange capacity (IEC) of 2.57–3.03 mequiv. g⁻¹ were prepared by the reaction of PBOS_x-r-PSBOS_y with 4,4′-methylene-bis[2,6-bis(hydroxyethyl)phenol] (MBHP) as the cross-linker in the presence of methanesulfonic acid. The cross-linked PBOS₂-r-PSBOS₈ membrane with the IEC of 3.03 showed a high proton conductivity compared to that of Nafion 117 over the entire range of 30–95% RH. Furthermore, the cross-linked membranes had a good mechanical strength even in the dry state as shown by DMA and a high oxidative stability against Fenton's reagent regardless of their high IECs. These results indicated that the introduction of flexible alkylsulfonated side chains to the polystyrene main chains improved the oxidative stability probably by preventing or reducing the hydroxyl radical attack on the polymer main chain, and also increased the water uptake and the proton conductivity at a low relative humidity due to the well-developed phase separation, which was supported by TEM observations.