Properties and PEFC durability of aromatic /aliphatic polyimide electrolyte membranes

In order to produce a proton conductive and durable polymer electrolyte membrane for fuel cell applications, a series of sulfonated polyimide electrolytes (SPI-5) containing aliphatic groups both in the main and the side chains have been synthesized. SPI-5 with the ion exchange capacity of 1.78 - 2.33 meq/g were obtained by a typical polycondensation reaction as transparent, ductile and flexible membranes. The proton conductivity of SPI-5 was slightly lower than that of the perfluorinated ionomer (Nafion) membrane below 100°C, but comparable at higher temperature and humidity. By incorporating a small amount of trifunctional monomer, the proton conductivity and hydrolytic stability of SPI-5 were improved. It was found out that block copolymers showed much higher proton conductivity than the random copolymers. Well-developed hydrophilic and hydrophobic phase separation was confirmed for the block copolymer membranes, which should be responsible for high proton conductivity. Fuel cell was fabricated with SPI-5 membrane and operated at 80 °C and 0.2 A/cm² supplying hydrogen and air both at 60% or 90% RH. SPI-5 membrane showed comparable performance to Nafion and durable for 5000 h without distinct degradation.