Sulfonated polyimides bearing benzimidazole groups for direct methanol fuel cell applications

Sulfonated polyimides (SPIs) bearing benzimidazole groups in the main chains were synthesized from 1,4,5,8-naphthalenetetracarboxiylic dianhydride, 4,4′-bis(4-aminophenoxy)biphenyl-3,3′-disulfonic acid and 2-(p- or m-aminophenyl)-5-aminobenzimidazole in m-cresol. The resulting polymer solutions were not precipitated to gain the solid polymers but directly cast into films because the precipitated SPIs became insoluble in common organic solvents. The strong interaction between sulfonic acid and benzimidazole groups reduced the water uptake and methanol uptake. The SPI membranes with ion exchange capacities (IECs) of 1.9-2.0 mequiv./g displayed reasonably high mechanical strength, thermal stability, water stability and proton conductivity. They showed anisotropic membrane swelling in water with 2.8 times larger swelling in thickness direction than in plane one and anisotropic proton conductivity with 25% smaller conductivity in thickness direction than in plane one. They suppressed methanol crossover in direct methanol fuel cell (DMFC) operation and displayed fairly high DMFC performances even at high methanol concentrations. The Faraday's efficiency and overall DMFC efficiency at 60 °C and 200 mA/cm² for SPI membrane with IEC of 1.9 mequiv./g were 75 and 23%, respectively, at 5 wt% methanol feed concentration and 39 and 10.2%, respectively, at 20 wt%. The SPI membranes have high potential for DMFC applications at mediate temperatures (40-80 °C).