Perfluorinated polymer membranes with sulfonic acid groups such as DuPont™ Nafions are commonly employed as proton conducting membranes of polymer electrolyte membrane fuel cells (PEMFCs), due to their good mechanical and chemical properties, excellent proton conductivity, and long-term durability). However, because of drawbacks of high cost and low performance at high temperature, alternative membranes such as sulfonated poly(ether sulfone)s, poly(ether ketone)s, poly(phenylene)s, polyimides, and polybenzimidazoles have been actively investigated. However, the highly sulfonated membranes have problems in dimensional stability and mechanical strength due to their excess water uptake.

One simple approach to overcome such problems associated with excessive water uptake is to crosslink the polymer membranes. Various crosslinking methods have been employed for aromatic hydrocarbon membranes. For example, polymer membranes have been crosslinked via esterification, UV irradiation, Freidel?Craft reaction, and electron beam irradiation. Most of the crosslinked membranes exhibited improved dimensional stability, though each method can have some advantages and disadvantages.
Among various crosslinking reactions, electrophilic aromatic substitution reactions (Friedel-Craft alkylation or acylation) appear most useful.
The electron beam-induced crosslinking method seems to be also very convenient due to the fast process, deep penetration of the electron beam, and the lack of need for an initiator and high reaction temperature. However, most crosslinking agents used for this purpose have ester bonds that can be hydrolyzed under acidic and high temperature conditions, which are similar to the operational conditions of fuel cells.
In this study, sulfonated poly(ether ether ketone) (SPEEK) was very efficiently crosslinked via a Friedel-Craft reaction using 1,6-dibromohexane and AlCl3. The resulting crosslinked SPEEK (c-SPEEK) membranes exhibited improved dimensional stability, thermal and chemical stability, and mechanical strength with slight reduction in the elongation. The methanol permeability was reduced by approximately two orders of magnitude by the crosslinking reaction. The proton conductivities of c-SPEEK membranes were greater than Nafion-212 in the temperature range of 30?90 ℃.
In this study, the SPEEK membranes were also crosslinked via electon beam irradiation, using a non-hydrolizable crosslinking agent, 1,6-bis(4-vinylphenyl)hexane (BVPH), which was synthesized in this laboratory. Compared to the pristine membrane, the crosslinked SPEEK membranes exhibited significantly improved dimensional stability, chemical stability, and mechanical strength. The crosslinking procedure slightly reduced the proton conductivity of the membranes. The crosslinking of SPEEK with BVPH was also found to slightly reduce the proton conductivity of the membranes, but significantly reduced the methanol permeability.
Overall, both of the current methods can be conveniently and efficiently applicable to most aromatic hydrocarbon polymer membranes. However, when the crosslinked membranes were immersed in 1.0 M HCl aqueous solution, they did not maintain their orginal shapes due to partial dissolution. Thus further studies are needed to improve such stability problem.