Aromatic ionomers based on 2,8-dioxydibenzofuran as a post-polymerisation, sulfonatable unit: synthesis, fabrication of thin-film membranes and performance evaluation in a direct-methanol fuel cell

"ABSTRACT: Linear polycondensation of activated aromatic dihalides including 4,4'-difluoro-diphenylsulfone, 1,3-bis(4-fluorobenzoyl)benzene and 3,3'-bis(4-fluorobenzoyl)biphenyl with a combination of the two bisphenol monomers 4,4'-dihydroxybenzophenone and 2,8- dihydroxydibenzofuran affords a novel series of high molecular weight aromatic polyethers. Post-polymerisation sulfonation of these copolymers in 96% sulfuric acid yields a corresponding series of ionomers in which sulfonation has occurred at the 3- and 7-positions of the dibenzofuran rings. The ion-exchange capacities of these ionomers were in the range 1.3 to 1.8 mmol g-1, and their molecular weights (Mn) by GPC were between 39 and 56 kD with dispersities (Đ) between 2 and 3. All the ionomers were successfully solution-cast into tough, transparent, thin-film membranes (40 - 90 μm) which were evaluated for direct- methanol fuel cell (DMFC) performance in terms of proton conductivity, methanol diffusion coefficient, limiting current density and maximum power density. A dibenzofuran-based ionomer derived specifically from 3,3'-bis(4-fluorobenzoyl)biphenyl showed DMFC performance equivalent to, or even slightly better than, a control membrane produced from the industry-standard fluorocarbon ionomer Nafion 115."