Cross-linked porous polybenzimidazole (PBI) membranes were prepared by mixing a low-molecular-weight compound (porogen) and a cross-linker with the polymer to form cross-linked polymer membranes in order to increase the mechanical strength and proton conductivity. SEM images of the cross-section of the porous polymer membranes show the pore size and morphology. The cross-linking by p-xylylene dichloride can effectively improve the mechanical properties of the porous PBI membranes after phosphoric acid doping. The CpPBI-60 membrane, which is doped with 9 moles of phosphoric acid, has a tensile modulus of 0.45 GPa. The good mechanical strength of the cross-linked porous PBI membranes makes them able to hold more phosphoric acid and, consequently, have higher proton conductivity. Fenton's test indicated that the covalently cross-linked structure played an important role in the radical oxidative stability of the porous membranes. The doping level of phosphoric acid in the cross-linked porous PBI membranes showed that the enhanced conductivity was due to the increase of porosity, which results in the increase of acid uptake. Impedance analysis showed that the conductivity of the cross-linked porous PBI membranes could reach 2.1 × 10⁻² S cm⁻¹ at 160 °C under anhydrous conditions.

A novel asymmetric polybenzimidazole (PBI) membrane used for high temperature proton exchange membrane fuel cells has been successfully fabricated by a soft-template method using the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) as the porogen. Typically, the asymmetric PBI membrane exhibits a double-layer structure comprising a dense layer and a porous layer with a distinguishable boundary. The morphology and asymmetry of the porous structure have been characterized by SEM micrographs. The density difference between the polymer matrix and the porogen can be considered as the driving force for developing the asymmetrical structure. Phosphoric acid-doped asymmetric PBI with a high porosity exhibited considerably enhanced doping levels and proton conductivity. For example, a doping level of up to 23.6 and a proton conductivity as high as 6.26 × 10⁻² S cm⁻¹ were achieved. Moreover, the crosslinking modification of asymmetric PBIs had beneficial effects on the mechanical strength and oxidative stability, which were investigated. We have also demonstrated the fuel cell performance of a membrane electrode assembly (MEA) based on the asymmetric PBI at elevated temperatures under anhydrous conditions in the present work.