Methanol permeability and proton conductivity of sulfonated co-polyimide membranes

Two types of sulfonated co-polyimides (co-SPIs) with ion exchange capacities of 1.83–2.32 meq/g were prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), two types of sulfonated diamines, namely, 4,4′-bis(4-sulfophenoxy)biphenyl-3,3′-disulfonic acid (BAPBDS) and bis(3-sulfopropoxy)benzidines (BSPBs), and common non-sulfonated diamines via statistical polycondensation reaction. The BSPB-based co-SPI membranes had a clear microphase-separated structure composed of hydrophilic and hydrophobic domains but the connection of hydrophilic domains was rather poor. On the other hand, the BAPBDS-based co-SPI membranes did not show such a clear microphase separation. The co-SPI membranes except for a few ones showed high proton conductivities of 0.10–0.16 S/cm in water at 323 K, which was comparable to that of Nafion 112 (0.13 S/cm). The methanol permeabilities of the co-SPI membranes hardly depended on feed composition up to 50 wt.% of methanol and were in the range of 0.5 × 10−6 to 1.8 × 10−6 cm2/s at 323 K, which were more than two times smaller than those of Nafion 112. As a result, the co-SPI membranes showed more than two times larger ratios of proton conductivity over methanol permeability than Nafion, suggesting high potential for direct methanol fuel cell application.