Phosphoric acid-doped cross-linked porous polybenzimidazole membranes for proton exchange membrane fuel cells

Abstract

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.