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

Abstract

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 °C (73.4–82.2 mS cm-1) but moderate swelling behaviors (i.e., in-plane 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 °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 °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.