Ether spaced N-spirocyclic quaternary ammonium functionalized crosslinked polysulfone for high alkaline stable anion exchange membranes

Abstract

N-spirocyclic cations containing anion exchange membranes (AEMs) possess excellent alkaline stability, but poor mechanical and conductive properties due to the rigid N-spirocyclic structure and lack of flexible side chain structural design. In this work, a flexible ether spaced N-spirocyclic quaternary ammonium functionalized side chain is proposed, in which hydroxyl substituted N-spirocyclic cation provides active grafting site with chloromethylated polysulfone through Williamson etherification. The flexible ether-oxygen spacer promotes aggregation of the rigid spirocyclic cations and thus significantly improves conductivity. The uniformly in-situ thermal crosslinking between benzene rings and the residue chloromethyl groups in polysulfone greatly enhances mechanical properties of the membranes. Constrained ring conformation of N-spirocyclic cation, ether spaced benzyl-free side chain, as well as the OH− blocking crosslinked networks endow an excellent alkaline stability to the membranes. The ether spaced N-spirocyclic containing membranes exhibit well micro-phase separation structure, up to around 85.7 mS cm−1 of hydroxide conductivity at 80 oC, around 10.5 MPa of tensile strength in hydrated state and around 95.6% conductivity retention after immersion in 1 M KOH at 80 oC for 720 h. The performance is among the best N-spirocyclic functionalized AEMs reported in the literature.