Paul Majsztrik

Diffusion and Interfacial Transport of Water in Nafion

Water absorption, membrane swelling, and self-diffusivity of water in 1100 equivalent weight Nafion were measured as functions of temperature and water activity. Free volume per water at 80 °C, determined from water uptake and volume expansion data, decreases with water content in the membrane from 12 cm(3)/mol at λ = 0.5 H(2)O/SO(3) to 1.5 cm(3)/mol at λ = 4. The change in free volume with water content displays a transition at λ = 4. Limiting water self-diffusivity in Nafion was determined by pulsed gradient spin echo NMR at long delay times. The limiting self-diffusivity increases exponentially with water activity; the rate of increase of diffusivity with water content shows a transition at λ = 4. The tortuosity of the hydrophilic domains in Nafion decreased from 20 at low membrane water activity to 3 at λ = 4. It suggested a change in the connectivity of the hydrophilic domains absorbed water occurs at λ ∼ 4. The diffusivity results were employed to separate the contributions of diffusional and interfacial resistance for water transport across Nafion membranes, which enabled the determination of the interfacial mass transport coefficients. A diffusion model was developed which incorporated activity-dependent diffusivity, volume expansion, and the interfacial resistance, and was used to resolve the water activity profiles in the membrane.

Water Permeation through Nafion Membranes: The Role of Water Activity

The permeation of water through 1100 equivalent weight Nation membranes has been measured for film thicknesses of 51-254 microm, temperatures of 30-80 degrees C, and water activities (a(w)) from 0.3 to 1 (liquid water). Water permeation coefficients increased with water content in Nafion. For feed side water activity in the range 0 < a(w) < 0.8, permeation coefficients increased linearly with water activity and scaled inversely with membrane thickness. The permeation coefficients were independent of membrane thickness when the feed side of the membrane was in contact with liquid water (a(w) = 1). The permeation coefficient for a 127 microm thick membrane increased by a factor of 10 between contacting the feed side of the membrane to water vapor (a(w) = 0.9) compared to liquid water (a(w) = 1). Water permeation couples interfacial transport across the fluid membrane interface with water transport through the hydrophilic phase of Nafion. At low water activity the hydrophilic volume fraction is small and permeation is limited by water diffusion. The volume fraction of the hydrophilic phase increases with water activity, increasing water transport. As a(w) --> 1, the effective transport rate increased by almost an order of magnitude, resulting in a change of the limiting transport resistance from water permeation across the membrane to interfacial mass transport at the gas/membrane interface.

An instrument for environmental control of vapor pressure and temperature for tensile creep and other mechanical property measurements

An instrument for measuring the creep response of a material maintained under a controlled environment of temperature and vapor pressure is described. The temperature range of the instrument is 20-250 degrees C while the range of vapor pressure is 0-1 atm. Data are presented for tests conducted on this instrument with Nafion, a perfluorinated ionomer developed by DuPont and used as a membrane in polymer exchange membrane fuel cells, over a range of temperature and water vapor pressure. The data are useful for predicting long-term creep behavior of the material in the fuel cell environment as well as providing insight to molecular level interactions in the material as a function of temperature and hydration. Measurements including dynamic and equilibrium strain due to water uptake as well as elastic modulus are described. The main features of the instrument are presented along with experimental methodology and analysis of results. The adaptation of the instrument to other mechanical tests is briefly described.

Mechanical and Transport Properties of Nafion: Effects of Temperature and Water Activity

Recent studies have shown that water absorption changes the mechanical and transport properties of Nafion by orders of magnitude. The unusually large changes in properties are indicative of microstructural changes induced by water absorption. The experimental findings of changes in proton conduction, water transport, elastic modulus, and stress relaxation are highlighted and explained by microphase segregation of hydrophilic domains resulting from water absorption. Water absorption is proposed to cause clustering of hydrophilic sulfonic acid groups and water within a hydrophobic polytetrafluoroethylene matrix. The hydrophilic domains form a network that facilitates transport and create physical cross-links that stiffen Nafion. At high temperature and low water activity, the entropy of de-mixing breaks the clusters apart, causing a large drop in elastic modulus of the polymer and a large decrease in the rates of water and proton transport.

Water Sorption, Desorption and Transport in Nafion Membranes

Water sorption, desorption, and permeation in and through Nafion 112, 115, 1110 and 1123 membranes were measured as functions of temperature between 30 and 90 ◦ C. Water permeation increased with temperature. Water permeation from liquid water increased with the water activity difference across the membrane. Water permeation from humidified gas into dry nitrogen went through a maximum with the water activity difference across the membrane. These results suggested that the membrane was less swollen in the presence of water vapor and that a thin skin formed on the dry side of the membrane that reduced permeability to water. Permeation was only weakly dependent on membrane thickness; results indicated that interfacial mass transport at the membrane/gas interface was the limiting resistance. 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.