Resonance Stabilized Perfluorinated Ionomers for Alkaline Membrane Fuel Cells

Abstract

Perfluorosulfonic acids such as Nafion are industrial standard cation exchange ionomers for polymer electrolyte membrane fuel cells because of their high gas permeability, hydrophobicity, and inertness to electro-chemical reaction. In this research, pentamethylguanidinium functionalized, perfluorinated hydroxide conducting ionomers for alkaline membrane fuel cells were prepared and characterized. The alkaline stability of the ionomers largely depended on the adjacent group that connected the cation; Sulfone guanidinium functionalized ionomer degraded almost completely after soaking in 0.5 M NaOH at 80 °C for 24 h, while phenylguanidinium functionalized ionomer did not degrade under the same conditions for 72 h. Spectroscopic data and density functional theory calculation suggested that the stability of the phenylguanidinium ionomer was greatly improved by charge delocalization of the formed resonance structure. Alkaline membrane fuel cells using the resonance stabilized perfluorinated ionomer in the catalyst layers on quaternized polyphenylene membrane showed excellent performance (ca. maximum power density = 466 mW/cm2) and promising stability (ca. Tafel slope degradation rate = 225 μV/dec h) at 80 °C under H2/air conditions.