(379c) Chemical Degradation of Proton Exchange Membranes in Fuel Cells

Perfluorinated sulfonic acid (PFSA) membranes were developed in mid-1960s. The chemical stability, permselectivity, and high proton conductivity has made these ionomer membranes attractive for a broad range of applications, such as fuel cells and electrolysis. PFSA consists of a polytetrafluoroethylene (PTFE) backbone and pendent fluorodiether side chains with sulfonic acid end groups. It is produced by copolymerization of a perfluorinated vinyl ether comonomer with tetrafluoroethylene. In recent years, it has been established that for fuel cell applications, the degradation of the MEA (membrane electrode assembly) occurs under low humidity conditions, whereas no obvious degradation was observed under high humidity conditions. Membrane failure can be caused by thermal degradation, mechanical stress, and chemical attack especially under accelerated conditions such as low humidity and high temperature.

Controlling the activity of water in the reactant streams is critical both to the design of fuel-cell systems and to the useable life of membrane separators. In this study, fuel-cell durability tests were conducted under different levels of relative humidity. The emission rates of various degradation products such as HF, SO42- and TFA (trifluoroacetic acid) were determined as a function of water activity. The degradation of the membrane was accelerated as the level of water activity is reduced. The membranes become less conductive, more brittle and rigid after fuel-cell testing. ATR-FTIR investigations showed that the decomposition of the ether group in the middle of side chain corresponds well with the detection of a TFA product. Thermogravimetric analysis also showed a decrease in thermal stability after testing at lower humidity. Formation of cracks was observed in membranes degraded under conditions of low humidity.

A model for H2O2 formation, transport, and reaction in PEMFCs is established for the first time. Profiles of oxygen and H2O2 concentration inside the fuel cell are simulated using the agglomerate model for the electrode. The predicted concentration of H2O2 shows the same trend as experimental data under different conditions, but the level was only of the same magnitude. Low humidity, high temperature, and high oxygen/hydrogen partial pressures were found to increase the concentration of H2O2. An increase in membrane thickness or metal ion contaminant level reduces the concentration of H2O2 in the membrane. Lowering the oxygen permeability in the ionomer is the most important and effective method to reduce the formation of H2O2. The simulation results also show little change in H2O2 concentration while operating the fuel cell above 0.6 V. Anodes designed with considerable thickness, high catalyst loadings and active areas can also help to suppress H2O2 formation. A further extension of the model of membrane degradation based on the main chain unzipping process indicates that the mechanism changes with water activity.