Preparation and properties of the cross-linked sulfonated polyimide containing benzimidazole as electrolyte membranes in fuel cells

Sulfonated polyimides containing benzimidazole (SPIBI) with varies sulfonation degree (DS) are synthesized by 1, 4, 5, 8-naphthalenetetracarboxylic dianhydride (NTDA), 5-amino-2-(4-aminophenyl) benzimidazole (APABI) and 4, 4′-diaminodiphenyl ether-2, 2′-disulfonic acid (ODADS). The cross-linked SPIBI membranes can be prepared with 3, 3′, 5, 5′-tetramethyl-diphenyl diglycidyl ether (TMBP) as cross-linker during the following thermal treatment process. By adjusting the DS of SPIBI and the weight ratio of the crosslinker, a series of the cross-linked membranes are obtained. All the obtained membranes exhibit good cross-linking densities for the gel fractions higher than 75%. Compared to pure SPIBI membranes, the mechanical properties, oxidative and hydrolytic stabilities of the cross-linked membranes are improved significantly. For instance, the tensile strength of the cross-linked membranes is in the range of 53.6-108.4 MPa, the anti-free oxidative stability of SPIBI-80-15 (with a cross-linker content of 15 wt% and DS of 80%) is almost four times of the data of the uncross-linked SPIBI-80 which is tested by Fenton′s reagent (3% H2O2, 3 ppm Fe2+) at 80 °C. SPIBI-80-15 also exhibits much higher hydrolytic stability for more than 1440 h, while that of SPIBI-80 is 27 h in the same condition. The proton conductivity of the cross-linked membranes ranges from 10−3 to 10−2 S cm−1.