Tuning hydrogen bond and flexibility of N-spirocyclic cationic spacer for high performance anion exchange membranes

Abstract Although N-spirocyclic cation exhibits superior alkaline stability, its rigidity leads to low conductivity and mechanical stability in anion exchange membranes (AEMs). Herein, N-spirocyclic cationic side chain with novel -NH-(CH2)x-NH- spacer is proposed to promote hydrogen bond networks and flexible aggregation of ionic clusters in AEMs. Chloromethyl N-spirocyclic cation is synthesized and bonded with diamines of different lengths to achieve benzyl free N-spirocyclic side chain, as well as to tune the length of the side chains. Molecular dynamic simulation suggests that amino groups induce hydrogen bond networks with water to broaden hydrophilic pathways for hydroxide ions conduction. Trade-off between diffusion coefficient and solubility parameter of the side chain is proposed to demonstrate the side chain-backbone interaction and thus the effects of spacer length on the aggregation of ionic clusters. It coincides well with the experimental results that PSF-C6-ASD with -NH-(CH2)6-NH- spacer exhibits the largest ion clusters (about 8.5 nm) and the highest OH− conductivity (107.1 mS cm−1 at 80 °C), as well as excellent elongation at break (38.3%) and alkaline stability (tolerance to 8 M hot KOH). The properties are better than that of the previously reported N-spirocyclic based AEMs.