Temperature dependence of water content and proton conductivity in polyperfluorosulfonic acid membranes

Abstract

The Fourier transform infrared (FTIR) spectra and the proton conductivity were determined in the temperature range of 25°C to 100°C, while in contact with a water-saturated nitrogen atmosphere. The water content of the membrane was not continuously proportional to the mole fraction of water in the vapor phase but reached a minimum near 70°C. The proton conductivity was found to maximize between 55 and 65°C. These results indicated that membrane water content is not as important as temperature in determining membrane conductivity. At temperatures below 55°C, the conductivity increased with temperature, despite a decrease in membrane water content. In this temperature domain membrane water played a less significant role in determining the conductivity than the effect of temperature in overcoming the 3 to 5 kcal/mol activation energy for proton movement. Above 70°C the conductivity decreased, while the membrane water content increased. The infrared results showed an increase in the number of un-ionized sulfonic acid groups at higher temperatures. The decrease in conductivity was attributed to this increase in the quantity of sulfonic acid groups that were not ionized. These results are explained using current models of cluster formation. Cluster formation is favored by the sulfonic acid/water dipole interactions but is hindered by extension of the polymer side chains as the sulfonic acid groups aggregate. The side chain configuration contributes to the entropy term for the free energy of cluster formation. Hence clusters will become less stable at higher temperatures and some sulfonic acid groups withdraw from the clusters into the nonpolar fluorocarbon region.