Studies of new ionomers for polymer electrolyte membrane (PEM) fuel cells

At the center, both figuratively and literally, of a polymer electrolyte membrane (PEM) fuel cell lies the polymer membrane that separates the reaction gases and conducts protons from the anode to the cathode. The membrane is required to exhibit long-term stability, good mechanical properties, and high ionic conductivity. These properties are exhibited by Nafion, a commercially-produced perfluorosulfonic acid ionomer that is the current industry standard.; Novel ionomers that substitute the sulfonic acid functional group of Nafion with the bis[(perfluoroalkyl)sulfonyl]imide superacid functional group have been synthesized by DesMarteau and coworkers at Clemson University. This dissertation describes work aimed at characterizing sulfonyl imide ionomers through water absorption tests, ionic conductivity measurements, and single-cell tests.; The water absorption and ionic conductivity of the ionomers were found to be strongly dependent upon relative humidity of the environment and ionomer equivalent weight. Water absorption and ionic conductivity decreased with decreasing relative humidity, with the severity of the decrease being strongly dependent upon the ionomer's equivalent weight. Ionic conductivity was also found to be dependent on the chemical structure of the side chain and on the polymerization conditions.; Ionomers were incorporated into membrane electrode assemblies (MEAs) as membranes and also into electrode structures for single-cell tests. In general, MEAs fabricated using sulfonyl imide membranes performed as well as or only slightly worse than MEAs fabricated using Nafion membranes. Though slight improvements in performance were observed with the use of monoacid sulfonyl imide ionomers, these could usually be accounted for when the performance was corrected for differences in membrane thicknesses. Ionomer chemical structure was found to affect performance as monoacid sulfonyl imide ionomers exhibited better performance, particularly at higher current densities, than diacid ionomers.; The performance effects of operating PEM fuel cells with low relative humidity gas feed streams were investigated. Under low humidity, large fluctuations were observed in cell current and resistance coupled to a significant decrease in overall cell current. These effects were reversed when the gas feed streams were rehydrated. A cyclic flooding and dehydration of the anode electrode is proposed as a possible cause of the observed performance fluctuations and decreases.