The SO2 cross-linked membranes of sulfonated polyimides (SPIs) bearing sulfophenoxy side groups were prepared and evaluated as polymer electrolyte membranes for polymer electrolyte fuel cells (PEFCs). They maintained a high mechanical strength and proton conductivity after aging in water at 130°C for 500 h, indicating their high water stability. PEFCs with the SPI membranes showed high performances at 90°C under a high feed-gas humidity of 84% relative humidity (RH), and also fairly high performances even at a low feed-gas humidity of 30% RH due to the back-diffusion of water formed at the cathode. The PEFCs were operated under a constant current density of 0.5A/cm2 at 90°C and 84% RH for 1600 h without any reduction in cell performance. There was no change in the Fourier transform infrared spectra for the SPI membranes before and after the durability test. These results indicate that they have a high durability in PEFC operation. The cross-linked SPI membranes have a high potential for PEFCs at higher temperatures above 80°C .

Sulfonated polyimides (SPIs) bearing sulfophenoxy side groups were successfully prepared. Their uncross-linked and cross-linked membranes were evaluated as polymer electrolyte membranes for polymer electrolyte fuel cells (PEFCs). They maintained high mechanical strength and high proton conductivity after aging in water at 130 o C for 500 h, indicating their high water stability. PEFCs with the SPI membranes showed high performances at 90 o C and a high humidity (85 %RH) of supplied gases. They also showed fairly high performances even at a low humidity of 30%RH due to the back diffusion of water formed at the cathode. PEFCs with the cross-linked SPI membranes were operated under a constant current density of 0.5 A/cm 2 at 90 o C and 85 %RH for 1600 h without any reduction in cell performance, indicating their high fuel cell durability. The SPI membranes have high potential for PEFCs at higher temperatures above 80 o C.