Chemically modified proton-conducting membranes based on sulfonated polyimides: Improved water stability and fuel-cell performance

A series of branched/crosslinked sulfonated polyimide (B/C-SPI) membranes were prepared and evaluated as proton-conducting ionomers based on the new concept of in situ crosslinking from sulfonated polyimide (SPI) oligomers and triamine monomers. Chemical branching and crosslinking in SPI oligomers with 1,3,5-tris(4-aminophenoxy)benzene as a crosslinker gave the polymer membranes very good water stability and mechanical properties under an accelerated aging treatment in water at 130 degrees C despite their high ion-exchange capacity (2.2-2.6 mequiv g(-1)). The resulting polymer electrolytes displayed high proton conductivities of 0.2-0.3 S cm(-1) at 120 degrees C in water and reasonably high conductivities of 0.02-0.03 S cm(-1) at 50% relative humidity. In a single H-2/O-2 fuel-cell system at 90 degrees C, they exhibited high fuel-cell performances comparable to those of Nafion 112. The B/C-SPI membranes also displayed good performances in a direct methanol fuel cell with methanol concentrations as high as 50 wt % that were superior to those of Nafion 112. (c) 2006 Wiley Periodicals, Inc.