N-cyclic quaternary ammonium-functionalized anion exchange membrane with improved alkaline stability enabled by aryl-ether free polymer backbones for alkaline fuel cells

Abstract

To realize highly stable anion exchange membrane for alkaline fuel cells, aryl-ether free polystyrene (PS) was employed as polymer backbone and functionalized with alkaline stable N-cyclic quaternary ammonium (QA), e.g. six-membered (DMP) and bis-six-membered N-cyclic QA (ASU) cations via “click chemistry”. Surprisingly, aryl-ether free PS backbone having ASU cation displayed excellent chemical resistance in 1 M NaOH CD3OD/D2O solution at 80 °C even for 3000 h without obvious degradation, as confirmed by NMR spectroscopy. After blending with poly(styrene-ethylene-co-butylene-styrene) (SEBS) copolymer which improved the film-forming ability due to the poor mechanical properties of PS, the ASU-based blended membrane (SEBS/PS-ASU) having SEBS polymer of 10 wt% possessed the hydroxide conductivity of 31.6 mS/cm with an ion exchange capacity (IEC) of 1.76 meq./g. This value was higher than the blended membrane having DMP cations (25.8 mS/cm with a IEC of 1.92 meq./g). Similar behavior has been observed for the PPO system having the same functional cations. It was believed that the high water uptake of ASU-based membrane resulted in the higher ion conductivity. Remarkably, 92.4% of initial hydroxide conductivity was retained for SEBS/PS-ASU membrane with aryl-ether free PS backbone after storage in 1 M NaOH at 80 °C for 900 h, indicating the higher alkaline stability over the AEM counterpart with aromatic PPO polymer backbones having aryl ether bonds (21.0% retention in conductivity under the same test conditions). And the accelerated stability test in 5 M NaOH at 80 °C demonstrated that only 16.8% of loss in conductivity was observed after 1800 h of testing for the blend AEMs having ASU cations. Furthermore, as ionomers in catalyst layers, the ASU-functionalized PS copolymers showed a peak power density of 130 mW/cm2 at a current density of 210 mA/cm2.