| A series of copolyimides bearing sulfonic acid group were synthesized and characterized as solid polymer electrolytes for PEM fuel cells. These copolymers consist of long rigid rod, liquid crystal segments bearing sulfonic acid groups, connected by angled or linear bulky comonomers as spacers.; The basic hypothesis behind the molecular design is: the rigid rod segments tend to align in nematic liquid crystal packing; angled or bulky spacers force chains further apart than in the homopolymer, generating unoccupied volume in the form of pores lined with sulfonic acid groups. Water is absorbed and retained, increasing proton conductivity at low relative humidities compared with polymers that pack “well”. This molecular design was proposed based on the analysis of existing proton conducting polymers and aimed at overcoming the major drawbacks of the well known material—Nafion ®.; X-ray diffraction, water uptake, TGA and proton conductivity measurements showed that copolymers with angled or bulky spacers have larger inter-chain spacings, higher water uptake capacity and better water retention capability, and therefore higher proton conductivities than the homopolymer, especially at low relative humidities. At room temperature, the copolyimides' conductivities are the same or higher than Nafion® throughout the whole relative humidity region. At the low relative humidity end, 15%RH, the best material conducted 13 time higher than Nafion®. In addition, in contrast to Nafion® whose conductance was almost independent of temperature over 2580°C at 100% RH, these copolymers show increasing conductivity with increasing temperature, up to ∼0.8S/cm at 100% RH, 75°C. The other physical properties of such polyimides are also satisfying: they have lower hydrogen and methanol permeabilities and higher Young's moduli than Nafion®.; The major problem that these polyimides face is their water stability. Polyimide homopolymer and copolymers with low mole ratio of comonomers behave poorly in contact with liquid water or high temperature water vapor. Several physical and chemical modification methods were used to improve the water stability. The structural changes involved were analyzed and compared. The best method yielded materials that were stable in boiling water for 10 days and had higher proton conductivity than Nafion® at 80°C. |
