Cross-Linked Polybenzimidazole Membrane for PEM Fuel Cells

Abstract

Despite several unique advantages, high-temperature proton-exchange membrane fuel cells (HT-PEMFCs) based on polybenzimidazole (PBI) membranes suffer from various drawbacks like weak chemical resistance, poor mechanical strength, acid leaching, etc., which eventually reduces the performance of the cell. In order to minimize these drawbacks and to improve the cell performance, in this work we have developed proton-exchange membranes (PEMs) in which pyridine-bridged-oxypolybenzimidazole (PyOPBI) and brominated polyphenylene oxide (BrPPO) were chemically cross-linked by an ex situ methodology. Three cross-linked membranesP1P2, andP3consisting of 12.5, 25.0, and 37.5 wt % BrPPO, respectively, with respect to PyOBI were successfully fabricated and PEM properties were studied. These membranes showed much improved acid stability, oxidative stability, mechanical strength, and strong resistance to swelling in concentrated phosphoric acid (PA) solution. They were found to be completely stable in 85% PA whereas uncross-linked PyOPBI membranes readily dissolved in 60% PA. The reason for such stability has been ascribed to the cross-linked network structure of the membrane. TheP1membrane exhibited remarkably high proton conductivity (0.123 S cm-1) whereas the pristine PyOPBI membrane showed a conductivity of 0.008 S cm-1at 180oC. The single cell measurement under anhydrous conditions at 160 °C of membrane electrode assembly (MEA) obtained from theP1membrane displayed good fuel cell efficiencies with power density 290 mW cm-2and current density 848.7 mA cm-2at 0.3 V whereas under the identical measurement conditions, MEA of the pristine PyOPBI membrane showed 96.4 mW cm-2power density and 321.5 mA cm-2current density at 0.3 V. All these results indicated that cross-linked membranes have great potential to be used in the HT-PEMFC.