Sulfonated polyimides with flexible aliphatic side chains for polymer electrolyte fuel cells

A series of sulfonated polyimides (SPIs) were synthesized from a new side group sulfonated diamine, 2,2′-bis(4-sulfobutoxy) benzidine monomer (2,2′-BSBB), 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), and common nonsulfonated diamine monomers by polycondensation reaction. Membranes were prepared by casting their m-cresol solutions. The SPI membranes displayed proton conductivity σ values of 0.2–0.4 S cm−1 at 140 °C in liquid water and showed stronger relative humidity (RH) dependence of proton conductivity compared to that of Nafion 112. Most of the SPI membranes were both tough and flexible and thermally stable up to 230–250 °C. XRD spectra suggested the presence of crystalline structure in the membranes. TEM analysis indicated clear microphase separation structure between hydrophobic main chain and hydrophilic side chain. The fully humidified SPI membranes exhibited obvious anisotropic dimensional changes with much larger expansion in thickness than in plane. The long term proton conductivity stability (ca. 500 h) was confirmed at 140 °C. The methanol permeabilities (PM) of the SPI membranes were in the range of 4.0 × 10−7–1.2 × 10−6 cm2/s with a 50 wt% methanol in feed at 30 °C, which were one order of magnitude smaller than that of Nafion 112. As a result, the membranes displayed much larger selectivity towards both high proton conductance and low methanol permeation, compared to Nafion 112, suggesting high potential application for direct methanol fuel cells (DMFCs).