Hexyl quaternary ammonium- and fluorobenzoyl-grafted SEBS as hydrophilic–hydrophobic comb-type anion exchange membranes

Abstract

The new hydrophilic–hydrophobic comb-type anion exchange membrane (AEM) was prepared from SEBS grafted with hexyl quaternary ammonium (HQA) and fluorobenzoyl. As confirmed by XRD and TEM, a better phase separation was obtained by adding a highly hydrophobic fluorine-substituted pendant as an enhanced hydrophobic spacer to the conducting head group (which contained a hexyl spacer as a hydrophilic unit). The HQA-F1-SEBS and HQA-F5-SEBS membranes, with fluorobenzoyl and pentafluorobenzoyl pendants, respectively, showed enhanced hydrophobicity compared to their non-fluorinated counterpart (HQA-SEBS). The highest hydroxide ion conductivity (87.0 in water and 14.37 mS cm-1 at 95% RH) was obtained at 80 °C with HQA-F5-SEBS, which exhibited the most thermodynamic incompatibility between hydrophilic and hydrophobic units. The mechanical properties (tensile strength and Young's Modulus) of the HQA-F5-SEBS membrane were also improved; they were almost 2.5 times higher than those of the HQA-SEBS. The HQA-F5-SEBS showed a current density of 500 mA cm-2 at a potential of 0.6 V and a peak power density of 354 mW cm-2. It also showed stable-cell durability for up to 100 h. Compared with the typical hydrophilic comb-type SEBS membrane and the previously reported SEBS-based AEM, its cell performance was significantly higher.