Durable and highly proton conducting poly(arylene perfluorophenylphosphonic acid) membranes

Abstract

Phosphonated aromatic polymers show several important advantages as proton exchange membranes (PEMs), including high thermal and chemical stability. However, the conductivity generally needs to be significantly enhanced for most electrochemical applications. Here, we have prepared a series poly(p-terphenyl perfluoroalkylene)s functionalized with highly acidic perfluorophenylphosphonic acid by first carrying out triflic acid mediated polyhydroxylations involving p-terphenyl, 2,2,2-trifluoroacetophenone and perfluoroacetophenone. Subsequently, the resulting polymers were quantitatively and selectively phosphonated in the para positions of the pendant perfluorophenyl units by employing an efficient Michaelis-Arbuzov reaction. X-ray scattering of proton exchange membranes (PEMs) based on the phosphonated polymers showed efficient ionic clustering with the interdomain distance depending on the acid content of the polymer. Although the water uptake and swelling was moderate (even at high temperature) the PEMs showed high proton conductivity, up to 111 mS cm−1 at 80 °C fully hydrated, and reaching 4 mS cm−1 at 50% RH at the same temperature. This may be ascribed to the distinct phase separation and high acidity of the polymers. The stability of the PEMs was excellent with thermal decomposition only above ~400 °C. Moreover, no change in weight, appearance or molecular structure was detected after 5 h immersion in Fenton's reagent at 80 °C, demonstrating an excellent chemical resistance of the PEMs towards free-radical attack. The radical resistance of the present phosphonated PEMs was found to increase with the acid content, which is in contrast to corresponding sulfonated PEMs. The combination of high thermochemical stability and high conductivity implies that the present materials are attractive for use as ionomers in catalyst layers and as PEMs in fuel cells and water electrolyzer applications.