(534f) Water Sorption, Desorption And Transport In Nafion Membranes

Water sorption, desorption, and permeation in and through Nafion 112, 115, 1110 and 1118 membranes were measured as functions of temperature between 30 and 80ºC. Water permeation was measured from liquid water or humidified nitrogen into dry nitrogen. Water permeation from liquid water into dry nitrogen increased with temperature and also increased with the nitrogen flow rate; the largest permeation rate was at the highest nitrogen flow rate where the water activity difference across the membrane was greatest. At 30º the water permeation was nearly independent of membrane thickness indicating interfacial mass transport controlled the permeation. At 80ºC the permeation rate decreased with increasing membrane thickness, but the decrease in permeation was less than expected for a diffusion limited process. Water permeation from humidified gas into dry nitrogen went through a maximum with nitrogen flow rate, going to zero at high nitrogen flow rates where the water activity difference across the membrane was a maximum. These results indicated that the membrane was less swollen in the presence of water vapor and that a thin skin must form on the dry side of the membrane that reduces permeability to water. The diffusivity of water in Nafion deduced from water sorption into a dry Nafion film was almost two orders of magnitude slower than the diffusivity determined from permeation experiments. The rate of water sorption did not scale with the membrane thickness as predicted by a Fickian diffusion analysis. The results indicated that water sorption was limited by the rate of swelling of the Nafion. Water desorption from a water saturated film was an order of magnitude faster than water sorption. Water desorption appeared to be limited by the rate of interfacial transport across the membrane/gas interface. The analysis of water permeation and sorption data identifies different regimes of water transport and sorption in Nafion membranes corresponding to diffusion through the membrane, interfacial transport across the membrane-gas interface and swelling of the polymer to accommodate water.