Cross-linked proton exchange membranes for direct methanol fuel cells: Effects of the cross-linker structure on the performances

A highly sulfonated poly (ether ether ketone) with pendant amino groups (Am-SPEEK) has been synthesized. The resulting copolymer showed good solubility in common organic solvents. Then the pendant amino groups of the Am-SPEEK were utilized to react with two kinds of cross-linker, epoxy resin and dibromide, to yield various cross-linked membranes. After cross-linking, the membranes could not dissolve in common solvents, just became swollen. In generally, all the cross-linked membranes showed improved mechanical properties and high dimensional stabilities, whereas the uncross-linked membranes highly swollen or even dissolved in water at high temperature. The proton conductivity of the membranes increased with an increase in temperature. At 80 °C, all the cross-linked membranes showed high proton conductivity, in the range of 0.111–0.140 S cm−1. Especially, the TMBP cross-linked membrane showed a proton conductivity of 0.140 S cm−1, which was higher than that of Nafion 117 (0.125 S cm−1). The membranes with high proton conductivity, good oxidative stability, and improved methanol residence have been successfully developed. Furthermore, the influence of different main chain of the cross-linker on the performance of the cross-linked membranes was also investigated.

► Water soluble Am-SPEEK with active sites was synthesizes successfully.
► Epoxy resin and dibromide with different structures were used as cross-linkers.
► After cross-linked, all the membranes showed improved performance.
► Flexible chain made a less dense cross-linking network than that of rigid chain.
► TMBP-cross-linked membrane with rigid main chain showed the best performance.