Sulfonated Copolyimides in Ionic Liquid Media for Fuel Cell Application

In recent years, significant attentions have been paid to develop high-performance cost-effectivepolymer membranes for polymer electrolyte membrane (PEM) fuel cells as clean energy sources.Among different candidates, perfluorosulfonate ionomers especially Nafion are the most commoncommercially available PEMs [۱]. However, some disadvantages such as high crossover, lowoperating temperature, and the high cost critically limit their industrial application [۲]. Due to thesedeficiencies widespread efforts have been devoted developing alternative PEMs based on sulfonatedaromatic polymers. In this regard, synthesis, and properties of a novel designed copolyimide usingionic-liquid medium (۱-ethyl-۳-methyl imidazolium bromide) was conducted in this research. Thesulfonated diamine (BANBPDS) and nunsulfonated diamine (BANBP) were synthesized first (Scheme۱). Then polycondensation reaction of ۱,۴,۵,۸-naphthalene tetracarboxylic dianhydride (NTDA) withcombination of BANBPDS and BANBP in adjusted molar ratios resulted in preparation of sulfonatedcopolyimides with five sulfonation degrees of ۴۰, ۵۰, ۶۰, ۷۰, and ۸۰. In this way sulfonatedcopolyimides containing carbonyl, flexible ether, and naphthyl bulky groups with high-molecularweightwere prepared. Investigation of the properties showed the capability of the prepared membranesfor fuel cell performance [۳].The results of the investigation of thermal stability of the sulfonated polyimides are reported in Table۱. All the sulfonated copolyimide exhibited higher Tg than Nafion and had a two-step degradation patternin TGA analysis. The first step of degradation that was observed around ۳۰۰°C corresponded to the degradation of sulfonated groups. As the sulfonated parts of the copolyimidesare the weak units from the thermal stability point of view in these polymers, by increasing thesulfonation content of the copolymers, the mass loss at the first step was increased and also thedegradation temperature decreased. The second weight loss was attributed to the decomposition of thepolyimide main chain which showed thermal stability of the polyimide units up to ۴۳۰ °C. The datashowed that these polymers can be used in high temperature PEMs.The inherent viscosity, ion exchange capacity (IEC), and water uptake of copolyimides wereinvestigated. The IEC values directly depend on the content of the sulfonated groups in the polymer.On the other hand, high amorphous nature of BANBPDS-DADO copolyimides led to more wateruptake and consequently higher IEC values. Also, these polymers showed reasonable protonconductivity that made them suitable candidates as membrane for fuel cell.