•Sufonated polyimides containing benzimidazole group (SPIBI) are synthesized.•The cross-linked membranes are prepared by SPIBI and diglycidyl ether.•The cross-linked membranes exhibit improved hydrolytic and oxidative stability.•The cross-linked membranes show improved mechanical property.•The cross-linked membranes are proved to be a promising material in FCs.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.

A facile crosslinking method of polybenzimidazole (PBI) with sulfonyl azide groups (sPBI-SA) for proton conducting membranes is proposed. Thermally crosslinkable sPBI-SA is synthesized from sulfonated PBI (sPBI) and sodium azide. The structures of sPBI, sPBI-SA, and crosslinked sPBI are confirmed by FTIR and 1H NMR. Upon heating, sPBI-SA loses nitrogen and form nitrene, which reacts with CH-bond of the backbone of another chain of PBI via the reactions of hydrogen abstraction, recombination, or CH-bond insertion. The crosslinking structure of PBI membranes is thus formed. Compared with the uncrosslinked membranes, the crosslinked sPBI membranes exhibit improved tensile strength, migration stability of phosphoric acid (PA), dimensional stability, and chemical oxidative stability. Whereas, the doping ability of PA and the proton conductivity of the crosslinked membranes decrease a little.