High Hydroxide Ion Conductivity with Enhanced Alkaline Stability of Partially Fluorinated and Quaternized Aromatic Copolymers as Anion Exchange Membranes

For enhancing hydroxide ion conductivity, alkaline stability, and fuel cell performance of quaternaized aromatic/perfluoroaklyl copolymer (QPAF) membranes, ammonium groups attached to the polymer backbone have been investigated. The ammonium groups included dimethyl-butylamine (DMBA), dimethylhexylamine (DMHA), and 1,2dimethylimidazole (DMIm) groups in comparison to the trimethylammonium (TMA) group. DMBA turned to be the optimum ammonium group for QPAF membranes in terms of its high hydroxide ion conductivity based on well-connected and larger phase-separated morphology than that of QPAF-TMA with similar ion exchange capacity (IEC) value. QPAF-DMBA (IEC = 1.33 mequiv g(-1)) exhibited the highest hydroxide ion conductivity among the tested membranes up to 152 mS cm(-2) in water at 80 degrees C, which was 1.6 times higher than that of QPAFTMA (95 mS cm(-1)). In addition, QPAF-DMBA exhibited reasonable alkaline stability in 1 M KOH at 60 degrees C for 1000 h. The remaining conductivity was 44 mS cm(-1) (58%) for QPAF-DMBA, while that for QPAF-TMA was 1.0 mS cm(-1) (1%). QPAFDMBA (IEC = 1.09 mequiv g(-1)) exhibited excellent stability in 1 M KOH at 80 degrees C without change in the ion conductivity (22 mS cm(-1)) for 500 h. The post-test membranes exhibited a minor degradation in QPAF-DMBA as suggested by FT-IR spectra and DMA analyses. An H-2/O-2 fuel cell was operated with the QPAF-DMBA membrane to achieve the maximum power density of 167 mW cm(-2) at the current density of 0.42 A cm(-2), which was higher than that (138 mW cm(-2)) for QPAF-TMA membrane under the same operating conditions.