Highly Stable and Conductive Multicationic Poly(biphenyl indole) with Extender Side Chains for Anion Exchange Membrane Fuel Cells

In order to develop a high-performance and long-term stable anion exchange membrane (AEM), ether-bond-free poly(biphenyl bromohexyl indole) (PPHIN) was prepared and explored as a highly alkaline stable polymer backbone. Combining two alkyl chain modification strategies, a series of flexible double-cationic side chains with different lengths of extender chains were grafted onto the PPHIN backbone to improve the hydrophilic/hydrophobic microphase separation, ionic conductivity, and alkaline stability of the AEM. The resulting PPHIN-N1C possessed a high hydroxide conductivity of 136 mS/cm at 80 °C due to the well-developed microphase separation. Furthermore, the PPHIN-N8C with a long extender chain exhibited high ionic conductivity (103 mS/cm), a low swelling ratio, and excellent alkaline stability. The ionic conductivity of PPHIN-N8C only decreased by 13% after soaking in 2 M NaOH at 80 °C for 1000 h, which was attributed to the steric hindrance of the extender hydrophobic alkyl chain. The single cell using the PPHIN-N8C membrane has a maximum peak power density of 216 mW/cm2 at a current density of 472 mA/cm2 at 80 °C. The results suggest that this type of PPHIN-based AEM is promising in anion exchange membrane fuel cell application.