Self-cross-linked quaternary phosphonium based anion exchange membranes: assessing the influence of quaternary phosphonium groups on alkaline stability

Abstract

A series of novel quaternary phosphonium functionalized poly(aryl ether sulfone)s were successfully synthesized using a new methyl containing monomer via a multi-step process including bromination, quaternary phosphination and alkalization. The study of the chemical stability of these alkaline anion exchange membranes (AAEMs) using different spectroscopic techniques (¹H NMR, ³¹P NMR and ATR-FTIR) revealed the excellent alkaline stability of the polymer backbone and the partial degradation of phosphonium groups after alkaline exposure at 60 °C for 288 hours. Self-cross-linked membranes with different cross-linking degrees were also synthesized via Friedel–Crafts electrophilic substitution C-alkylation. Despite the fact that the IEC values of the cross-linked membranes are very low, the nano-phase separated morphologies observed by TEM enable the formation of ionic clusters, thus facilitating hydroxide conductivity. Indeed, the normalized IEC hydroxide conductivity for self-cross-linked AAEMs is 13.1 mS g cm⁻¹ meq.⁻¹ at 70 °C, which is comparable to that of other cross-linked AAEMs. The results of ATR-FTIR spectroscopy, TGA, and IEC demonstrated the excellent alkaline stability of the self-cross-linked AAEMs even after 40 days of immersion in 1 M KOH at 60 °C or even under harsher conditions such as 4 M KOH at 80 °C for 16 days, suggesting that the self-cross-linking strategy was beneficial for the protection of phosphonium groups from hydroxide attack.