(330k) New Sulfonated Copolymer Membranes For High Temperature Proton-Exchange Membrane Fuel Cells

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

Recently, we have synthesized sulfonated polyimide (SPI) copolymer membranes containing hydrophilic poly(ethylene oxide) (PEO) soft segments for water retention at high temperatures. The membrane showed similar fuel cell performance as Nafion® 112 at 70oC and 80% RH. However, the SPI membrane showed much better fuel cell performance than Nafion® 112 at 120oC and 50% RH, which was due to the improved water retention by PEO soft segments. We have also synthesized new sulfonated polybenzimidazole (SPBI)-based membranes. The membrane has exhibited a very high conductivity (> 0.1 S/cm) at high temperatures (> 120oC) and low humidities (even the anhydrous condition), which could essentially meet or exceed the DOE (Department of Energy) targets for PEM materials. Additionally, the newly synthesized SPBI-based membrane has possessed excellent thermal, oxidative, chemical, and hydrolytic stability even at high temperatures. Thus, it has the great potential for the high temperature and low humidity PEMFC application. All of these new membranes should be much more cost-effective since the starting materials are more than two orders of magnitude less expensive than those for Nafion® membranes.