Sulfonated Aromatic Polymers for Fuel Cell Membranes

Aromatic and heteroaromatic polymers are well known for their often excellent thermal and chemical stability as well as their good mechanical properties and high continuous service temperatures. Therefore, they have long been considered promising candidates for the development of proton-conducting membranes for fuel cells, especially for applications above 80 degrees C. Typically, sulfonic or phosphonic acid groups are introduced to provide acidic sites. While it is possible to introduce these groups by post-modification of the preformed polymers, the preferred method in many cases is modification of the monomers and subsequent polymer synthesis, because this allows better control of the number, distribution, and position of the acidic sites. Compared to perfluorosulfonic acid polymers, such as Nafion, proton-conducting membranes based on aromatic hydrocarbon polymers tend to exhibit excellent conductivities in the fully hydrated state and significantly reduced crossover, especially of methanol in DMFC applications. However, this often comes at the expense of a higher degree of swelling and a greater loss of conductivity with decreasing water content, which is considered a severe drawback e.g., for automotive applications. Recent approaches to improving the property profile of hydro-carbon membranes include block copolymers, rigid rod polymers, and the attachment of acidic groups via side chains.