Cross-linked norbornene sulfonated poly(ether ketone)s for proton exchange membrane

A new proton conducting membrane where the poly(ether ether ketone ketone) copolymer containing unsaturated norbornene unit and pendant sulfonic acid were synthesized by aromatic substitution polymerization reaction using 1,3-bis(4-fluorobenzoyl)benzene (DFBP), 6,7-dihydroxy-2-naphthalenesulfonate (DHNS), and 1,4-dihydro-1,4-methanonaphthalene-5,8-diol (NB-ph-diOH) monomers. The degree of sulfonation can be accurately controlled by adjusting the DHNS and NB-ph-diOH mole ratio in the feed stream. The increase of proton conductivity, water uptake and ion exchange capacity (IEC) are found to corroborate with increasing the DHNS feed concentration. Furthermore, cross-linking through the double bond in norbornene unit reduces both the methanol permeability and methanol solvent uptake. Current study shows that membrane with high proton conductivity and low methanol swelling can be optimized by balancing the degree of sulfonation (DS) with the degree of cross-linking through controlling DHNS (for optimized DS) feed concentration, and the DVB concentration (for optimized degree of cross-linking). The best membrane with high proton conductivity and low degree of swelling is NB-sPEEKK-80X. The DMFC single cell assembled using this membrane (85 μm thickness) displayed fair density reaching 16.5 mW/cm2 at 60 °C and 14.5 mW/cm2 at 50 °C.