Triblock ionomers derived from linear and hyperbranched poly(ether ketone)s: ionomer synthesis, fabrication of proton-transport membranes and evaluation for direct-methanol fuel cell applications

"Polycondensation of an aromatic ether-ketone AB2 monomer in the presence of bis(4-fluorobenzoyl)-terminated PEEK affords semi-crystalline triblock copolymers in which the central block is a readily-sulfonatable PEEK chain and the outer blocks are non- sulfonatable hyperbranched moieties. Post-polymerisation sulfonation of each these copolymers in 96% sulfuric acid, after extraction of a cyclic oligomer fraction originating from the AB2 monomer, yields the corresponding ionomer in which sulfonation has occurred exclusively and quantitatively on the central block. The ion-exchange capacities of these ionomers were in the range 1.4 to 1.8 mmol g-1, and their inherent viscosities were 3.9 to 4.8 dl g-1. The ionomers were successfully solution-cast into tough, transparent, thin-film membranes (ca. 40 μm in thickness) 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. The triblock ionomers showed substantially higher DMFC performance than a control membrane based on the industry-standard fluorocarbon ionomer Nafion® 115, and their methanol diffusion coefficients were two to three times lower."