Directly copolymerized partially fluorinated disulfonated poly(arylene ether sulfone) random copolymers for PEM fuel cell systems: Synthesis, fabrication and characterization of membranes and membrane–electrode assemblies for fuel cell applications

Abstract

The effect of partial fluorination on fuel cell behavior at 120 °C and lower (50%) humidity was investigated with hexafluoroisopropylidene bisphenol (Bisphenol AF or 6F) incorporation into a 45 mol% poly(arylene ether sulfone) random copolymer. The novel series of directly disulfonated poly(arylene ether sulfone) copolymers with various mole ratios of fluorination were synthesized following traditional nucleophilic aromatic substitution polycondensation reactions. The levels of the fluorinated bisphenol in the statistical copolymer architecture were varied from 0 to 100 mol% using hexafluoroisopropylidene bisphenol (6F) and the correct stoichiometric amount of 4,4′-biphenol. The disulfonation mole percent was held constant at 45% ( e.g. mole ratio of 3,3′-disulfonated-4,4′-difluorodiphenyl sulfone [SDFDPS] to 4,4′-difluorodiphenol sulfone (DFPDS) of 0.45:0.55). The constant disulfonation afforded precise understanding of the parameters that partial fluorination improved, while allowing for enhanced protonic conductivity associated with 45 mol% disulfonated copolymers. Solution cast transparent membranes were characterized using intrinsic viscosity (lithium bromide modified NMP), gel permeation chromatography, Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance ( 1H NMR), X-ray photoelectron spectroscopy (XPS), water uptake, protonic conductivity and hydrogen/air fuel cell experiments. High molecular weight copolymers were obtained based on intrinsic viscosity and gel permeation chromatography experiments. Quantitative incorporation of the 6F monomer into the copolymer was monitored using FTIR and NMR analyses. During film casting, self-assembly of the fluorinated groups at the membrane interface was monitored using XPS analysis. The data indicate that the fluorine atoms preferentially arranged at the air surface, presumably due to the lower surface energy environment. Water uptake decreased with increasing incorporation of the 6F monomer, which suggests that the hydrophobic units aided in water management. Protonic conductivity decreased slightly as the amount of fluorination increased, which could be explained by the decrease in the ion-exchange capacity (milli-equivalents of sulfonic acid moieties per gram of copolymer) due to the larger molar weight of the 6F repeat unit relative to 4,4′-biphenol (about 1.9–1.4). High temperature (120 °C) hydrogen/air fuel cell experiments indicated better Nafion ®-bonded electrode adhesion for the partially fluorinated materials, as depicted by lower high frequency resistance values obtained at 0.5 V.