Contaminant absorption and conductivity in polymer electrolyte membranes

During its lifetime, the polymer electrolyte membrane of a fuel cell may be exposed to numerous impurities originating from sources such as materials used in various components of the fuel cell stack and contaminants in the coolant and reactant gas. The contamination of the membrane by cationic impurities has detrimental effects on membrane properties with regard to conductivity, water management and durability. Thus the measurement of absorption of contaminants into the membrane and the investigation of its relation to these detrimental effects is of interest. Samples of Nafion 117 polymer electrolyte membrane were soaked in deionised water solutions containing part per million (ppm) concentrations of cation impurities, ranging from 0.1 to 100 ppm. Upon removal of the membranes from the soaking solutions, conductivity of the membranes and the effect of different cationic impurity concentrations on the conductivity was measured by impedance spectroscopy methods. Using atomic absorption spectrophotometry, the concentration of the cationic impurity species remaining in the soaking solutions was determined and the extent of ion absorption by the membranes calculated. Energy dispersive X-ray analysis was also employed and confirmed the presence of the contaminant ions in the membrane. Impedance studies exhibited a loss of conductivity, while an increase in ion absorption into the membrane was observed, when membranes were soaked in solutions of higher contaminant concentration. In this study, the capacity for ion absorption into the membranes was determined and the extent of membrane contamination was compared and correlated with the loss in conductivity of the membrane for different levels of contamination.