Alkali-Stable Anion Exchange Membranes Based on Poly(xanthene)

Poly(xanthene)s (PXs) carrying trimethyl ammonium, methylpiperidinium and quinuclidinium cations were synthesized and studied as a new class of anion exchange membranes (AEMs). The polymers were prepared in a superacid-mediated polyhydroxyalkylation involving 4,4’-biphenol and 1-bromo-3-(trifluoroacetylphenyl)-propane, followed by quaternization reactions with the corresponding amines. The architecture with a rigid PX backbone decorated with cations via flexible alkyl spacer chains resulted in AEMs with high ionic conductivity, thermal and chemical stability. For example, hydroxide conductivities up to 129 mS cm⁻¹ were reached at 80 °C, and all the AEMs showed exceptional alkaline stability with less than 4% ionic loss after treatment in 2 M aq. NaOH at 90 °C during 720 h. Critically, the diaryl ether links of the PX backbone remained intact after the harsh alkaline treatment, as evidenced by both ¹H NMR spectroscopy and thermogravimetry. Our combined findings suggest that PX AEMs are viable for application in alkaline fuel cells and electrolyzers.