| Proton-exchange-membrane (PEM) fuel cells have high efficiency and fast start-up times. They can be used to power automobiles, buses, commercial buildings, residences, communication devices, and electronic equipment. In PEM fuel cells, hydrogen and oxygen react to generate water and electricity. Water management is critical to fuel-cell operation because, on the one hand, too much water slows oxygen transport to the reaction sites and, on the other, too little water causes low proton conductivity, both resulting in reduced power output.;Currently, Nafion is used as a proton-conducting membrane in fuel cells. The purpose of this study is to investigate water diffusion in Nafion, polymer relaxation, and the effect of thermal history on Nafion-water equilibria and proton conductivity.;Water-Nafion phase equilibria and proton conductivities were measured in two ways. First, Nafion was in contact with saturated water vapor. Second, Nafion was in contact with liquid water at the same temperature. At 29°C, for preboiled, vapor-equilibrated Nafion exposed to water activity = 1 and air pressures ranging from 0 to 0.96 bar, water content lambda = 23 +/- 1 moles H2O/moles SO3-. For the preboiled, liquid-equilibrated membrane, lambda = 24 +/- 2. The water content of predried Nafion at 1 atm and 30°C was lambda = 13.7 +/- 0.2 when vapor-equilibrated and lambda = 13.1 +/- 0.5 when liquid-equilibrated. A Nafion membrane originally boiled in water had much higher liquid- and 100% relative humidity vapor-equilibrated proton conductivities than the same membrane originally dried at 110°C and RH less than 2%. The liquid-equilibrated and 100%-RH vapor-equilibrated membrane conductivities were the same when the membrane had the same thermal history. The conductivity data were fit to a model, and the water content was determined at different temperatures. Predried membrane water content increased with temperature, and preboiled membrane water content changed slightly with temperature. Both water-sorption and proton-conductivity data do not exhibit Schroeder's paradox. These studies and previous results suggest that Schroeder's paradox is resolved when attention is given to the thermal history of the absorbing polymer.;The water content of vapor-equilibrated Nafion was measured in a range of water activities and temperatures. A water-sorption-activity relationship was developed for 30 and 50°C. The water content increased smoothly as water activity increased to 1, and the water content for 100% RH-vapor equilibrated was the same as liquid-equilibrated Nafion, indicating no Schroeder's Paradox.;Steady-state and transient diffusion studies were conducted. Measured steady-state diffusion coefficients of water in Nafion at 30°C ranged between 2x10-5 and 1x10-6 cm 2/s, within the range reported in the literature. Water diffusion limited water transport, not a film interface. In addition, the water flux approached zero as the relative humidity approached 100% RH on one side of the membrane with liquid water on the other side, thereby indicating there was no Schroeder's Paradox.;Transient water sorption was measured using gravisorption at 50°C. The water sorption into the polymer was a hundred times slower than steady-state water diffusion coefficients would predict. Transient water uptake was limited by polymer relaxation, not by water diffusion coefficients. A water-diffusion polymer-relaxation model was developed. It provided a semiquantitative fit of the experimental data. |
