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.
Highlights
It is generally accepted that proton exchange membrane fuel cell (PEMFC) presents an attractive alternative to traditional power sources, due to its simplicity, high efficiency and non-pollution
Zeta potential and particle sizes of PBI in dimethyl acetamide (DMAc)/ LiCl solutions and viscosities of PBI/ DMAc/ LiCl solutions vary with LiCl concentration in the solutions
These results suggest that there are interpolymer associations through hydrogen bonding of –N-H with -N=C- groups of imidazole functional groups in the PBI/DMAc solutions without mixing with LiCl
Summary
It is generally accepted that proton exchange membrane fuel cell (PEMFC) presents an attractive alternative to traditional power sources, due to its simplicity, high efficiency and non-pollution. One of the most successful PEM for PEMFC is Nafion (a trade name of Du Pont Co) It is a perfluorosulfonated ionomer using water as proton conducting carriers and has an excellent proton conductivity and fuel cell performance at temperatures below 90oC with high humidity. One of the most promising hydrocarbon polymer PEMs is phosphoric acid doped polybenzimidazole (PBI/ H3PO4) membrane. It was first successfully used in high temperature (140~180oC) PEMFC by Savinell and Litt group in early 1990 [5,6,7,8,9]. The PBI/PTFE composite membranes are prepared by impregnation porous PTFE support thin films in PBI solutions. We reported the PBI solutions properties, the preparation of PBI/PTFE composite membranes, and the application of composite membranes to PEMFC
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