Proton conductive cross-linked benzoxazine-benzimidazole copolymers as novel porous substrates for reinforced pore-filling membranes in fuel cells operating at high temperatures

•Proton conductive benzoxazine having sulfonic acid groups was synthesized.•Porous cross-linked benzoxazine-benzimidazole copolymer substrates were prepared.•Tunable hydrophilicity, porosity and proton conductivity of porous substrates.•Excellent physical stability and mechanical strength of pore-filling membrane.•Outstanding cell performance of MEA prepared with the pore-filling membrane.

Proton conductive porous substrates consisting of cross-linked benzoxazine-benzimidazole copolymers are developed for practical application of reinforced pore-filling membranes in polymer electrolyte membrane fuel cells operating at high-temperatures (>100 °C) and low relative humidity (<50% RH) conditions. The porous proton conductive substrates are prepared by casting solution mixtures of sodium 3-(4-sulfonatophenyl-3,4-dihydro-2H-1,3 benzoxazine-6-sulfonate (pS) and poly[2,2′-(m-phenylene-5,5′-bibenzimidazole] (PBI) with dibutyl phthalate (DBP) as a porogen, followed by subsequent stepwise heating to 220 °C and extraction of DBP from the P(pS-co-BI) films. The resulting porous substrates are found to have mechanically robust cross-linked structures, tunable hydrophilicity, and proton conductivity. A pore-filling membrane is prepared by impregnating the porous substrate with sulfonated poly(arylene ether sulfone) having the degree of sulfonation of 70 mol%. The pore-filling membrane exhibits much improved dimensional stability and mechanical strength compared to the linear sulfonated poly(arylene ether sulfone) membrane and its proton conductivity and cell performance are found to be superior to the pore-filling membrane prepared using the porous substrate based on cross-linked benzoxazine-benzimidazole copolymers without any proton conductive acid groups.

Graphical abstractDownload high-res image (273KB)Download full-size image