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Novel Sulfonated Polyimide Copolymers for High Temperature Proton-Exchange Membrane Fuel Cells

He Bai, Department of Chemical and Biomolecular Engineering, The Ohio State University, 125A Koffolt Labs, 140 West 19th Avenue, Columbus, OH 43210-1180 and W. S. Winston Ho, Department of Chemical and Biomolecular Engineering, The Ohio State University, 125A Koffolt Labs, 140 West 19th Avenue, Columbus, OH 43210-1180.

Great progress has been made recently on the development of proton-exchange membrane fuel cells (PEMFCs) for both mobile and stationary applications, particularly for fuel cell vehicles. Dupont's Nafion and other perfluorinated sulfonic acid membranes are currently popular to use with low temperature PEMFCs due to their high proton conductivity as well as desirable mechanical strength and chemical stability. However, some disadvantages, such as high cost and insufficient membrane performance at high temperatures and low humidities, have seriously limited the application of these membranes. High temperature operation can increase the anode's tolerable level of CO in the fuel and accelerate the reaction rates. Low humidity and high temperature (about 120oC) operations can facilitate the water management of the fuel cell system. Therefore, it is desirable for a PEMFC to operate at high temperatures (above 100oC) and low relative humidities (blow 50% RH). As a result, the development of competitive and less expensive PEMs that have good performances at high temperatures and low humidities is crucial for fuel cell applications.

Sulfonated polyimides (SPIs) have been shown to be promising materials for high temperature PEMs mainly because of their excellent mechanical and thermal properties as well as their chemical stability. In the present study, novel six-membered ring SPI copolymers were successfully synthesized. Hydrophilic soft segments of poly(ethylene oxide) (PEO) were copolymerized into the SPIs system to increase the water retention of the copolymers at high temperatures and low humidities. The relative ratio of the sulfonic acid-containing hard segments and the PEO-containing soft segments was controlled through variation of the molar ratio of 4,4'-diaminostilbene-2,2'-disulfonic acid (DSDSA) and diamine terminated poly(ethylene oxide) (PEO-diamine). Flexible, transparent, and mechanically strong free-standing membranes were successfully synthesized by the solution casting method. The novel soft segment-containing six-membered ring SPI copolymer membranes exhibited desirable thermal and hydrolytic stability. The conductivity measurements showed that the SPI copolymer membrane exhibited high proton conductivities, which were higher than those of Nafion 115 at high RH levels (> 50%) at both 70oC and 120oC. The results from the fuel cell performance testing showed that this SPI copolymer membrane had similar open circuit voltage (OCV) and membrane conductance as Nafion 112 at 70oC and 80% RH. However, at 120oC and 50% RH, the novel SPI copolymer membrane showed a lower OCV (0.72 vs. 0.92 V), but much higher membrane conductance than Nafion 112. As a result, the novel SPI copolymer membrane exhibited better fuel cell performance than Nafion 112 when the current density was higher than 0.32 A/cm2.