. APPLICATION OF CNTs IN PEM
PEM is one of the key components in PEFCs, which not only restricts the performance of fuel cell but also the overall cost. An ideal PEM should be proton conductive, facilitating the transport of proton from anode to cathode; but yet electron insulating, avoiding the occurrence of electrical short circuit- ing. It should also have low permeability to the feed- ing fuels, such as hydrogen and methanol, and high chemical stability to withstand the harsh chemical conditions that the fuel cell is undergoing under operation. Additionally, it needs to be mechanically strong enough to withstand the stress of fabrication and the dimension change (swelling and contrac- tion) due to the change of water content as well. Owing to the presence of sulfonic acid groups, perfluorosulfonate ionomer, such as Nafion (see Fig. 1 for its chemical structure), possesses high proton conductivity. And it is also relatively mechani- cally and chemically stable. Therefore, it seems to match most of the requirements as a PEM mate- rial, and is a promising candidate. Indeed, its appli- cations in fuel cells have been explored extensively in the past few years.