High temperature polymer electrolyte membrane fuel cell (HT PEMFC), which has the advantages of high tolerance to fuel impurities and simplified system according to no humidification, is currently being studied extensively. The phosphoric acid (PA) with proton conductivity at high temperature (150℃) is used as an electrolyte in HT PEMFC. Therefore, the affinity between membrane material and phosphoric acid and membrane durability are very important. At present, membrane material for HT PEMFC is mainly used the polymer based on polybenzimidazole (PBI). However, the PBI has the problems such as low molecular weight, low PA doping level, PA leakage during operation, and membrane durability. According to some studies, sulfonic acid groups and hydroxyl groups have high affinity for phosphoric acid. Therefore, PBI-based polymer electrolyte prepared from each monomer showed improved doping level, proton conductivity and performance. In this study, we tried to fabricate electrolyte membrane for HT PEMFC with improved properties and cell performance by simultaneously introducing these two functional groups.

For the synthesis of s-PBI/2OH-PBI random copolymer, monosodium salt of 2-sulphoterephthalic acid (STA), 2,5-dihydroxyterephthalic acid (2OH-TA) and 3,3'- diaminobenzidine were prepared with ratio of 1:0:1 (10:0), 0.5:0.5:1 (5:5), 0.4:0.6:1 (4:6). The condensation reaction was used for synthesis of copolymer. The polymer membranes were fabricated by direct casting method. The polymer membrane of the fabricated film type was placed in the humidity chamber at each hydrolysis condition. The degree of polymerization of s-PBI/2OH-PBI random copolymer was confirmed by measuring the torque of the polymer solution in real time. The characteristics of the prepared copolymer were analyzed by FT-IR and doping level. Finally, single cell performance according to monomer ratio and hydrolysis condition was evaluated by IV curve measurement.

The IV curves of s-PBI/2OH-PBI random copolymer were affected by monomer ratio and hydrolysis condition. First, the IV curves showed better performance as the monomer ratio of 2OH-TA increased in hydrolysis conditions of 50℃ and RH 50%. Despite the lowest doping level in the 4:6 monomer ratio, the highest cell performance in 4:6 was attributed to the improvement of the mechanical properties with the increase of polymerization degree. On the other hand, the IV curve at 4: 6 monomer ratio showed the best performance in RH 80% and 70% phosphoric acid bath condition. This confirmed that the polyphosphoric acid was only partially hydrolyzed in the RH 50% condition.