Synthesis and properties of alkaline stable pyridinium containing anion exchange membranes

Abstract

Despite the fact that pyridinium based anion exchange membranes for alkaline fuel cells suffer from low alkaline stability due to hydroxide attack in the ortho position and the subsequent irreversible oxidation to 2-pyridone, new copolymers consisting of aromatic polyethers bearing 2,6-position protected pyridinium units as hydroxide conducting groups in the backbone, have been designed. In particular, aromatic polyethers bearing N-methyl pyridinium as functional group have been prepared by copolymerization of the corresponding 2,6-bis(4-hydroxyphenyl)pyridine or 2,6-difluoropyridine with various monomers, followed by methylation and alkalization in aqueous KOH solution. The alkaline stability was confirmed by thermal stability tests (TGA) and ¹H NMR spectroscopy. It was shown that the methylation degree plays an important role in alkaline stability; membranes with low methylation degree, e.g., the 2,6-phenyl substituted pyridinium based ones, remain remarkably stable even after 456 h in 1 M KOH solution at 60 °C, whereas the homopolymer with a higher methylation degree, showed some initial degradation due to hydroxide attack in the para position during the first 168 h and then remained stable for at least additional 288 h under the same conditions. Furthermore, the choice of substituents in the 2,6-position is as well critical for pyridinium AEM stability: pyridinium based membranes bearing phenyl substituents show higher alkaline stability compared to the corresponding membranes containing oxy substituents, due to conjugation stabilization.