Poly(aryl piperidinium)‐Based AEMs Utilizing Spirobifluorene as a Branching Agent

Abstract The high‐performance anion exchange membranes (AEMs) are developed by incorporating 9,9′‐spirobifluorene as a 3D 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, the AEM is refined, achieving high conductivity (≈190 mS cm⁻¹ 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 pycnometer 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 1,000 h. With such a membrane, an excellent cell performance is achieved, with a current density of 11.2 A cm⁻² at 80 °C and 2 V in 1 m KOH, and 1.8 A cm⁻² at 80 °C and 2 V in pure water conditions. The in situ membrane stability test, conducted at a constant current density of 1 A cm⁻² for 500 h, showed no significant degradation.