Considerations of the morphology in the design of proton exchange membranes: Cross-linked sulfonated poly(ether ether ketone)s using a new carboxyl-terminated benzimidazole as the cross-linker for PEMFCs

A novel benzimidazole trimer (alkyl-BI) was synthesized using 3,3′-diaminobenzidine and succinic acid as monomers, and then a series of cross-linked membranes (c-SPEEK-Xs) were prepared by a heating method using the alkyl-BI as the cross-linker. We systematically studied the water uptake and swelling ratio, thermal and chemical stability, mechanical properties, proton conductivity, and methanol permeability of the cross-linked membranes as a function of different loading ratios of the alkyl-BI trimer. All of the properties of the cross-linked membranes were significantly improved over membranes consisting of unmodified SPEEK. In particular, the performance of the c-SPEEK-Xs membranes was compared to that of a cross-linked membrane, c-SPEEK-BI7, which was prepared by using rigid-BI as the cross-linker. The latter cross-linker was synthesized by using 3,3′-diaminobenzidine and isophthalic acid as monomers in order to investigate how cross-linkers with different structures affect the morphology and properties of the membranes. Transmission electron microscopy (TEM) analysis showed that c-SPEEK-BI7 and the c-SPEEK-6.5% possess different hydrophilic/hydrophobic two-phase separation morphologies, which have dramatic effects on the proton conductivity and methanol permeability.