Study on the Chemical Stabilities of Poly(arylene ether) Random Copolymers for Alkaline Fuel Cells: Effect of Main Chain Structures with Different Monomer Units

We developed a series of heterocyclic quaternary ammonium-type poly(arylene ether) (PAE) random copolymers with moieties of sulfone, ketone, hexafluoroisopropyl, isopropyl, phenolphthalein, or phenylene to identify differences in the physicochemical properties of the AEM due to polymer backbone structure containing electron-withdrawing groups (EWGs) or electron-donating groups (EDGs). The 1-methyl pyrrolidine (PYR)-PAE membranes containing EDGs exhibited higher hydroxide conductivity compared with PYR-PAE membranes with EWGs due to more distinctly separated ion transport sites, which was confirmed through AFM phase images. The ionic conductivity of all prepared membranes was greater than 89% after an alkaline stability test for 700 h in 2 M KOH at 70 °C, PYR-PAE membranes with EDGs higher than that of EWGs revealed stronger alkaline stability. In particular, the PYR-PAE-PhPh membrane retained the highest alkaline stability of 96.9% due to the steric hindrance effect. In fuel cell operation, the PYR-PAE-PhPh membrane representing EDGs showed a higher power density (109 mW cm^-2) than that of the PYR-PAE-BPHF membrane (89 mW cm^-2) and commercial AEM (Fumion-FAA-3, 30 mW cm^-2). On the basis of these results, we suggest that structural design of the backbone is a critical strategy to develop an AEM with remarkable electrochemical properties and alkaline stability for alkaline fuel cell applications.