Preparation of PBI/H3PO4-PTFE Composite Membranes for High Temperature Fuel Cells

Abstract

The poly(benzimidazole) (PBI)/ poly(tetrafluoroethylene) (PTFE) composite membrane was prepared by impregnating a porous PTFE thin film in a PBI solution N,N’-dimethyl acetamide (DMAc) solution mixed with LiCl. LiCl was used as a stabilizer to avoid aggregations of PBI molecules in the DMAc solutions. In this paper, we report a 2 mg/ml PBI/ DMAc/ LiCl solution with a [LiCl]/[BI] molar ratio of ~8.0 (i.e. the LiCl/PBI is ~ 1.1 in wt ratio, where [BI] is the concentration of benzimidazole repeat unit in the solution) has a lowest PBI polymer aggregations and thus a lowest solutions viscosity. The PBI membrane and PBI/PTFE composite membrane prepared from the PBI/DMAc/LiCl solution with a [LiCl]/[BI] molar ratio of ~8.0 were used to dop H3PO4 and prepare membrane electrode assemblies (MEA). The unit cell performances of these MEAs were carried out at 150oC. Owing to the high mechanical strength of porous PTFE, the thickness of PBI/H3PO4-PTFE composite membrane is allowed to be lower than that of a PBI/H3PO4 membrane. The lower thickness of PBI/H3PO4-PTFE membrane than that of PBI/H3PO4 membrane results in a lower resistance of PBI/H3PO4-PTFE than PBI/H3PO4. Thus the MEA prepared from PBI/H3PO4-PTFE has a better fuel cell performance than that prepared from PBI.