Fabrication of thermally cross-linked polybenzimidazole membranes using a di-azide cross-linker for high temperature proton exchange membrane fuel cells

Cross-linked poly (4,4′-(diphenyl ether)-5,5′-bibenzimidazole) (OPBI) membranes are fabricated via an in-situ casting process using Ethynyl-grafted OPBI as the matrix and 4,4′-diazido-2,2′-stilbenedisulfonic acid disodium salt tetrahydrate (DSDAD) as the cross-linker, for high temperature proton exchange membrane fuel cell applications. The Ethynyl-grafted OPBI is synthesized through an N-substitution reaction with propargyl bromide to introduce cross-linkable sites into the OPBI backbone. Cross-linking is achieved through azide-alkyne click reactions and the dimerization of ethynyl groups, resulting in membranes that exhibit enhanced oxidative stability and mechanical properties compared to the linear OPBI membrane. Additionally, these membranes demonstrate increased proton conductivity, attributed to the unique characteristics of DSDAD as the cross-linker, which forms multiple hydrogen bonds within the polymer matrix and generates triazole linkages through the cross-linking reaction. With these improvements in various properties, the C_E-OPBI-2, featuring an optimum degree of cross-linking attains a power density 27 % higher than that of the unmodified OPBI membrane.