Synthesis and properties of a novel side-chain-type hydroxide exchange membrane for direct methanol fuel cells (DMFCs)

A series of poly(ether ether ketone) anion exchange membranes with tunable IEC (Ion-exchange capacity) values are polymerized from a novel bisphenol monomer (4-methyl)phenylhydroquinone. These poly(ether ether ketone) copolymers contain methyl groups on the rigid side chain. The degrees of bromination and quaternary amination of the copolymers are controlled by manipulating the contents of methyl groups in the copolymers. Hence, the IEC tunability of the membrane is achieved. The resulting PEEK-Q-xx membranes show good dimensional stability. The highest swelling ratio (PEEK-Q, with an IEC value of 0.90 mequiv. g ^-1) is only 9.0% at 80 °C, at which temperature, the hydroxide conductivity of PEEK-Q membrane is 0.031 S cm ^-1. The methanol permeability values of the membranes are all below 1.5 × 10 ^-7 cm ² s ^-1, which are much lower than that of Nafion 117. These properties make the membranes good candidate materials for anion exchange membranes for alkaline direct methanol fuel cells.