A novel proton-conductive membrane with reduced methanol permeability prepared from bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO)

A series of proton-conducting membranes were developed for direct methanol fuel cell (DMFC) applications via sulfonation of bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) base membranes. Besides the low manufacture cost, the membranes exhibited an excellent control on methanol crossover and swelling, and a sound balance with high proton conductivities. These can be attributed to the inherent properties of membrane structures: (i) benzyl substitution with bromine, which imparted the membrane stronger hydrophobicity, (ii) cross-linking between BPPO chains, which enhances the dimensional stability and renders the membrane a dense texture, (iii) proper content of sulfonic acid groups, which guarantees the proton conductivity. An optimal membrane was obtained after investigating the effects of the bromination degree and sulfonation process on the performances of corresponding membranes, i.e., the membrane possesses the methanol permeability of 2.64 × 10−8 cm2/s and characteristic factor Φ value of 30 times higher than that of Nafion® 117. The sulfonation process should be controlled within a proper period of time and in mild sulfonation conditions so as to achieve a proton conductivity higher than 0.07 S/cm for potential applications in DMFC.