Rational molecular design of partly fluorinated fuel cell membranes with high proton conductivity under low-humidity conditions

Abstract

Research on proton exchange membranes (PEMs) is closely tied to the development of PEM fuel cells, and the need to overcome the shortcomings of perfluorosulfonic acid PEMs. One of the key challenges is to devise efficient molecular designs towards PEMs with sufficient durability and proton conductivity under reduced humidity. Here, we report on a series of partly fluorinated PEMs based on high-molecular weight poly(arylene tetrafluorophenylsulfonic acid)s, synthesized in polyhydroxyalkylations of perfluoroacetophenone and balanced mixtures of bipenyl and p-terphenyl. Sulfonic acid groups were then introduced on the pendant pentafluorophenyl groups of the resulting polymers through an efficient thiolation-oxidation procedure. The fluorine content of these aromatic polymers was approximately 1/6 of the Nafion® benchmark. Foldable flexible PEMs were produced by tape-casting and showed thermal stability up to 260 °C, as well as excellent radical resistance. The proton conductivity increased with the acid content, and the PEM based on merely biphenyl reached 250 mS cm⁻¹ at 120 °C under fully humidified conditions, exceeding Nafion® NR212 by a factor 1.6. Under 30 % relative humidity at 80 °C, the same PEM achieved an outstanding 50 mS cm⁻¹, surpassing Nafion® by a factor 2.3. With a considerably higher acidity and lower ion exchange capacity than typical sulfonated hydrocarbon polyphenylene PEMs such as Pemion®, and significantly lower fluorine content and higher conductivity than Nafion®, the characteristics of the present PEMs may offer distinct advantages for fuel cells operating under reduced humidity.