Improving the mechanical and methanol crossover properties of fluorinated sulfonic acid functional polyphosphazenes by blending with polyvinylidene difluoride for fuel cell applications

Abstract

Sulfonic acid functional polyaryloxyphosphazenes are attractive alternatives to perfluorinated sulfonic acid membranes for fuel cells due to their high oxidative and thermal stability features. However, they usually suffer from low mechanical properties which is a barrier to their highly sought application as a membrane in polymer electrolyte membrane fuel cells. In this study, we synthesized a series of novel sulfonic acid functional polyaryloxyphosphazenes which have various amounts of hydrophobic 4-trifluoromethylphenol and hydrophilic 4-hydroxybenzenesulfonic acid co-substituents on the polymer backbone. Although the resulting materials exhibited better mechanical properties compared to other sulfonic acid functional polyaryloxyphosphazenes available in the literature, they were required to improve mechanical properties to act as a standalone membrane material in a fuel cell. Therefore, polyvinylidene difluoride has been used to enhance the mechanical properties whilst it also provided a low methanol crossover to the resulting blend membranes compared to the commercial perfluoro sulfonic acid Nafion 117. The blend membranes exhibited good oxidative stability, low water uptake, and swelling behaviors. The water contact angle and morphological analysis showed that the blend membranes have high hydrophobicity and porous structures which might contribute to the proton conductivity. The proton conductivity of blend membrane samples which have over the 60% sulfonic acid functional polyaryloxyphosphazene constituent, was over 140 mS/cm which was higher than that of Nafion 117 under hydrated conditions at 50 °C. These membranes also showed over ten times lower methanol crossover and good selectivity compared to Nafion 117.