Bi-guanidinium-based crosslinked anion exchange membranes: Synthesis, characterization, and properties

Abstract

In the overall alkaline stability for guanidinium cations, the substituted mode has been proven to play a very important role. Free and bulky alkyl substituents such as ethyl groups in guanidinium cations can provide large steric hindrance to resist the OH− attack and achieve much higher stability than conventional methyl-substituted ones. To obtain high-dimensional and good alkaline stabilities of anion exchange membranes (AEMs), linking the stable guanidiniums through suitable crosslinked structures (with different spacer structures or lengths) is a feasible strategy. In this study, bi-guanidinium-based crosslinked AEMs were prepared by Menshutkin reactions using brominated poly(2,6-dimethyl-phenylene oxide) and bi-guanidine crosslinkers followed by a complete quaternization of the remaining guanidine. The prepared membranes possess a low swelling ratio (<15% even at 80 °C), good thermal stability, and excellent mechanical properties. Alkaline stability test results showed that the bi-guanidinium crosslinked AEMs have excellent resistance to alkaline solutions, and more than 90% cations remained after immersing in 1 mol L−1 of NaOH solution at 60 °C for 30 days. These results suggest that crosslinking structures are effective in restraining the excess swelling and that delocalized guanidiniums are promising cations for application in AEMs.