Synthesis and property of a novel sulfonated poly(ether ether ketone) with high selectivity for direct methanol fuel cell applications

Bisphenol monomer 4-carboxylphenyl hydroquinone (4C-PH) containing carboxyl groups was synthesized by diazotization reaction of p-aminobenzoic acid and 1,4-benzoquinone and subsequent reductive reaction. Copolymerization of bisphenol A, 4C-PH, sodium 5,5′-carbonylbis(2-fluorobenzene-sulfonate) and 4,4′-difluorobenzophenone at various molar ratios through aromatic nucleophilic substitution reaction resulted in a new sulfonated poly(ether ether ketone) containing pendant carboxyl groups (C-SPEEK). The structures of the monomer 4C-PH and copolymers were confirmed by FT-IR and 1H NMR. Flexible and transparent membranes with sulfonic and carboxylic acid groups as the proton conducting sites were prepared. The dependence of ion-exchange capacity (IEC), water uptake, proton conductivity and methanol permeability on the degree of sulfonation has been studied. The highest proton conductivity of 0.16 S cm−1 was obtained from C-SPEEK-4 (IEC = 1.44 mequiv. g−1) at 80 °C, higher than that of Nafion117 (0.12 S cm−1). Incorporating carboxylic acid groups into copolymers led to higher dimensional stability and lower water uptake. The methanol permeability coefficients of C-SPEEK membranes were in the range of 3.54 × 10−7 cm2 s−1 to 5.9 × 10−9 cm2 s−1, which were significantly lower than that of Nafion117 (1.61 × 10−6 cm2 s−1). Accordingly, the C-SPEEK membranes exhibited much higher selectivity (conductivity/methanol permeability coefficient) compared with Nafion117 membrane.