Conductivity of polymer electrolyte membranes by impedance spectroscopy with microelectrodes

The protonic conductivity of the polymer electrolyte membrane (PEM) in the PEM fuel cell is critical to the overall power density of the fuel cell system. The conductivity can be influenced by the presence of impurity cations in the membrane. By the use of electrochemical impedance spectroscopy with microelectrodes, the local conductivity of Nafion membranes, which had been exposed to part per million (ppm) concentrations of impurity cations, was evaluated. Inorganic impurity cations studied included Cu2+, Fe3+, Na+ and Ni2+. Membranes were immersed in sulphate salt solutions of these cations, prepared in distilled water. Conductivity values at 0.1, 1 and 10 ppm cation impurity level were found to vary little from values for the blank solution. However at 100 ppm, a significant decrease in conductivity was observed. At this higher concentration of impurity, the Ni2+ and Cu2+ contaminated membranes displayed lower conductivity than that contaminated by Na+. Meanwhile Fe3+ contaminated membrane had the lowest conductivity. That this decrease in conductivity was greater for cations of higher valence corresponds with the high affinity of the sulphonic acid sites in Nafion to multivalent foreign cations. The results illustrate the detrimental effect of small amounts of contaminants on conductivity in Nafion membrane.