The effect of humidity on the degradation of Nafion® membrane

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 of membrane degradation based on the main chain unzipping process indicates that the mechanism changes with water activity. Finally, the representative reaction pathways in each degradation scheme were postulated.