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 1 H 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.

Anion‐Exchange Membranes for Fuel Cells: Synthesis Strategies, Properties and Perspectives

“…To solve the problems faced by AEMs with quaternary ammonium groups, such as facile degradation and excessive swelling, there is a resurgence of exploring AEMs with functional groups different from quaternary ammonium. The ever reported functional groups different from quaternary ammonium for AEMs are shown in Figure 12, including imidazolium 241–261, substituted imidazolium 262–274, benzimidazolium 275, 276, 1,2,3‐triazoles 277, TMP and DMP 278, QPOH 279–283, guanidinium 284–294, morpholinium 292, 293, DABCO 294–302, tetrakis(dialkylamino)phosphonium 303, triarylsulfonium 304, methylated melamine 305, pyrrolidinium 306, pyridinium 307, 308, TQA 309, GQA 310 and morpholinium 311.…”

Synthesis and characterization of benzimidazolium-functionalized polysulfones as anion-exchange membranes

“…Alternative cationic groups like guanidinium ones have also used in PPO and PES membranes showing remarkably high ionic conductivity. Likewise, pyridinium based AEMs were prepared for alkaline fuel cells in spite of its low alkaline stability. Unlike these groups, imidazolium (Im) was proposed as a good alternative.…”

Mechanically Stable Poly(arylene ether) Anion Exchange Membranes Prepared from Commercially Available Polymers for Alkaline Electrochemical Devices

“…[22][23][24] There are some recent reports of alkaline stable AEMs containing pendant side chains or crosslinked pendant side chains, AEMs prepared from block copolymers and AEMs with the cation group imbedded along the linear polymer backbone. [25][26][27][28][29] In the case of AEMs prepared with side chains and from block copolymers, high alkaline stability was obtained and attributed to the phase-separation between the hydrophilic domains and hydrophobic domains, where the fixed cation groups in the hydrophilic domains remained well solvated. 25,26,29 Increasing the solvation of the cation group in the presence of the hydroxide ion enhances the stability of the cation.…”

“…30 Cation groups imbedded along the linear polymer backbones can be sterically hindered, mitigating hydroxide ion attack at these sites. 27,28 Poly(arylene ether) backbones are desirable for AEMs because of their high oxidative stability and their simple synthesis from commercially available polymers. 31 A major drawback to using poly(arylene ether) backbones (e.g., poly(arylene ether) sulfone (PSF) and poly(2,6-dimethyl 1,4-phenyle oxide (PPO)) for AEMs is that the AEMs become brittle in alkaline solutions over time.…”