Cross-linked hydroxide conductive membranes with side chains for direct methanolfuel cell applications

Abstract

A series of novel poly(ether ether ketone) copolymers containing methyl groups on the side chain were prepared based on a new monomer (3,4-dimethyl)phenylhydroquinone. Then a series of hydroxide exchange membranes with different IEC values were obtained through bromination and quaternary amination of the copolymers. By adjusting the contents of methyl groups in the copolymers, we could control the final structures of the membranes. The chemical structures of the monomers and copolymers were analyzed by ¹H NMR spectroscopy. After that, for the purpose of enhancing the dimensional stability and methanol resistance of the membrane, we prepared cross-linked membranes through a Friedel–Crafts reaction between bromomethyl groups and aromatic rings. The properties of the membranes related to fuel cell application were evaluated in detail. All the membranes showed good thermal and mechanical stabilities and conductivities. Moreover, the cross-linked membranes exhibit better dimensional stabilities and selectivities. Among those membranes, xPEEK–Q-100 showed a high conductivity (0.036 S cm⁻¹ at 80 °C), a low swelling ratio of 6.6% and a methanol permeation coefficient of 2.9 × 10⁻⁷ cm² s⁻¹. The outstanding properties indicated that the application of PEEK–Q-xx membranes in fuel cells was promising.