Tri-quaternized poly (ether sulfone) anion exchange membranes with improved hydroxide conductivity

A novel anion exchange membrane with tri-quaternized side chain has been developed in the present work. Di-cationic quaternary ammonium salt, 1-(4′-{1″-azonia-4″-azabicyclo [2.2.2] octyl} butyl) azonia-4-azabicyclo [2.2.2] octane (AABAA), is synthesized and used as tri-quaternization reagent. The tri-quaternized side chains enhance ionic interactions, which induce good micro-phase separation and excellent swelling resistance. Better aggregation of ionic clusters is observed in transmission electron microscopy (TEM) image in the tri-quaternized poly (ether sulfone) membrane (tri-QPESOH) compared to that in the mono-quaternized membrane (mono-QPESOH). Tri‐QPESOH membranes achieve a well-balanced performance between high hydroxide conductivity and dimensional stability. Especially, the tri-QPESOH membrane with IEC 2.31 mmol g−1 exhibits high hydroxide conductivity of 45.9 mS cm−1 at 25 °C and 130.9 mS cm−1 at 80 °C, and low swelling ratio of 21.5% even at 80 °C. Tri-QPESOH membranes also show improved thermal, mechanical and chemical stabilities. These observations suggest that the tri-cationic side chains provide a promising solution to the electrochemical-mechanical balance of anion exchange membranes.