Enhancement in the alkaline stability of poly(arylene ether ketone)-typed anion-exchange membranes via phenylene piperidinium blocks

In alkaline environments, the cationic groups and polymeric backbones of conventional poly(arylene ether ketone)-typed anion-exchange membranes (AEMs) tend to degrade. Herein, we report poly(phenylene piperidinium-co-arylene ether ketone)s (QPPPiAEK-x) with enhanced alkaline stability as potential AEM candidates. Specifically, a diphenyl piperidine precursor was synthesized and subjected to Ni(0)-catalyzed coupling polymerization with poly(arylene ether ketone) oligomers and followed by classic Menshutkin reaction. Membranes with an ion exchange capacity ranging from 1.85 to 2.24 meq g-1 showed high water uptakes (97.7%- 159.2%) and high OH- conductivities at 80 degrees C (73.4-82.2 mS cm-1) but moderate swelling behaviors (i.e., inplane swelling ratios of 28.0%-35.8%, and through-plane swelling ratios of 28.4%-56.2%). Morphological studies confirmed the presence of hydrophilic-hydrophobic microphase separation (ionic domain sizes of 17.9-22.3 nm) in all membranes, which was ascribed to the high feed content of phenylene piperidinium blocks. The H2/O2 single-cell test revealed the highest power output (187.7 mW cm-2) for the QPPPiAEK-2.0 membrane at 60 degrees C and 100% relative humidity. During the stability test, all membranes exhibited adequate ion conduction abilities. The most durable membrane retained 69.6% of its initial conductivity after 936 h in a 1 M NaOH solution at 80 degrees C. Results of ex-situ nuclear magnetic resonance spectroscopy revealed that approximately 9.7%- 20.6% of cationic decomposition was the main degradation pathway for the QPPPiAEK-x ionomers, whereas 2.6%-4.4% accounted for the cleavage of backbones. Our study provides a promising methodology for fabricating alkali-stable AEMs for fuel cell applications.