Poly(aryl piperidium)-based AEMs utilizing spirobifluorene as a branching agent

We here developed high-performance anion exchange membranes (AEMs) by incorporating 9,9'-spirobifluorene as a three-dimensional branching agent, addressing the common trade-off between ion conductivity and dimensional/mechanical stability. By fine-tuning the ratio of terphenyl to biphenyl and the amount of the branching agent, we refined the AEM, achieving high conductivity (approximately 190 mS/cm at 80 °C in 1 M KOH) with decent dimensional/mechanical properties, comparable to the recently reported state-of-the-art membranes. Investigations using gas pycnometry and atomic force microscopy demonstrated that spirobifluorene enhances the fractional free volume around the membrane's backbone and more precisely modulates the separation between hydrophobic and hydrophilic domains, thus boosting both ion conductivity and mechanical stability. This membrane also displayed excellent chemical stability, with negligible degradation at 80 °C in 1 M KOH over 1000 h. With such a membrane, we achieved excellent cell performance, with a current density of 11.2 A/cm² at 80 °C, 2 V.