Communications to the Editor Bull. Korean Chem. Soc. 2010, Vol. 31, No. 8 2141 DOI 10.5012/bkcs.2010.31.8.2141 Terminally-Crosslinked Sulfonated Poly(fluorenyl ether sulfone) as a Polymer Electrolyte Membrane for both PEMFC and DMFC Application Roshni Lilly Thankamony, Green Baek, Hyoung-Juhn Kim, † Sukwoo Nam, † and Tae-Hyun Kim * Department of Chemistry, University of Incheon, Incheon 406-840, Korea. * E-mail: tkim@incheon.ac.kr † Fuel Cell Research Center, Korea Institute of Science and Technology, Seoul 136-791, Korea Received May 25, 2010, Accepted June 23, 2010 Key Words: Polymer electrolyte membrane, Sulfonated poly(fluorenyl ether sulfone), Terminal crosslinking, Proton conductivity, Methanol crossover F S O O F + HO KO 3 S OH + HO OH K 2 CO 3 O S O O O KO 3 S O x S O O O 1. NaH, DMF 2. allyl bromide 1 N 3 O N 3 OH KO 3 S HO y O S O O O KO 3 S O x S O O O 2 O KO 3 S O y FBP (y) HQS (x) FPS (x+y) (x:y = 5:5) H b H a 1. film casting 2. heat 3 crosslinked PFES-50 Scheme 1. Schematic representation for the preparation of the crosslinked poly(fluorenyl ether sulfone) (PFES-50). Proton exchange membrane (PEM) is a main component of the proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) for transferring protons from anode to cathode as well as providing a barrier to the fuel gas between two electrodes. Some specific limitations for the well- known perfluorinated membranes including high cost, high me- thanol permeability, and loss of the preferable properties at high temperature (80 o C) demand a search for alternative PEMs. 1-3 Sulfonated aromatic polymers have been widely studied as one of the alternatives to Nafion ® due to their good physical properties and moderate proton conductivity. 4-6 These properties have been shown to depend on the degree of sulfonation of the aromatic polymer: a high degree of sulfonation improves the proton conductivity, whereas degraded physical properties and increased methanol crossover are inevitably obtained for the highly sulfonated polymers. Crosslinking has often been used as an efficient means to lower methanol crossover and to enhance properties of the polymers. 7-9 However, reduced proton conductivity is accom- panied for most cases mainly due to the loss of sulfonic acid groups during thermal curing for the covalently crosslinked polymers and low water absorption for the acid-base type cross- linked polymers. Only a few reports have, indeed, presented membranes with high proton conductivity and low methanol permeability, especially at enhanced temperature. 10,11 In a fusion of the benefits of both sulfonated poly(ether sul- fone)s and crosslinking, we have recently introduced a novel terminally-crosslinked polymer system. 12,13 Unlike a majority of the crosslinked membranes, our sulfonated polyarylenes were crosslinked only at the terminus of the polymer main chain by an azide-assisted thermal curing. The resultant terminally-cross- linked sulfonated poly(ether sulfone)s (PESs) displayed an exceptionally high proton conductivity especially at elevated temperatures, and yet maintained most of their structural integ- rity. Nevertheless, the rather high methanol permeability at in- creased temperature and low dimensional stability limit the use of our terminally-crosslinked membranes for DMFC application. We report herein a new terminally-crosslinked polymer having a fluorenyl moiety as a dimensionally stable and highly selective polymer electrolyte membrane for proton over me- thanol transport even at increased temperature. The hydroxy-terminated poly(fluorenyl ether sulfone) having fluorenylbiphenyl unit 1 was first synthesized by polyconden- sation of fluorenylbiphenol (FBP) and hydroquinone sulfonic acid (HQS) with fluorophenyl sulfone (FPS) with a slight excess of a dihydroxy monomer (Scheme 1). Following allylation of