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2328 SATTERFIELD ET AL. Water management is a significant challenge for polymer electrolyte fuel cell operation. To keep the membrane fully hydrated, it is essential to increase the pressure in the fuel cell above the water vapor pressure. The other hydration method—operating with fully humidified feeds— creates a situation in which water that has formed in the fuel cell can flood the gas flow channels. However, avoiding flooding by keeping the water activity below unity can dehydrate the membrane and reduce the proton conductivity. In addition, changes in the water activity result in dimensional changes of the polymer as the polymer absorbs water.9–14 Water sorption creates an internal pres- sure in the membrane that causes it to swell against the confinement of the electrodes.15 The amount of water sorption of the membrane is determined by a balance between the membrane swelling pressure and the applied pressure from the electrode pressing against the membrane. In addition, the mechanical properties of the polymer change as functions of the temperature and water content. Further complications can result from taking the polymer above its glass transition ($110 8C for dry protonated Nafion),16–19 which can cause polymer chain rearrangements, leading to structural changes in the membrane at the mo- lecular scale. These changes in the polymer prop- erties may lower the membrane stability, perform- ance, and lifetime.20–22 We have studied the dynamic performance of fuel cell startup from a dry state and the current response to changes in the load. We have observed a multistep change in the current; this suggests that the membrane swells as it absorbs water, which alters the membrane electrode interface. It has also been observed that increasing the applied pressure sealing the fuel cell causes the internal membrane electrode assembly (MEA) resistance to increase, and this has been attributed to the physi- cal confinement of the Nafion membrane in a fuel cell limiting the water absorbed.15,23 The addition of an inorganic material to a poly- mer membrane can alter and improve the physical and chemical polymer properties of interest [e.g., elastic modulus, proton conductivity, solvent per- meation rate, tensile strength, hydrophilicity, and glass-transition temperature (Tg)] while retaining the polymer properties important to enabling the operation in the fuel cell. A number of investiga- tors have examined composite membranes for use in polymer electrolyte fuel cells.7,8,24–35 The com- posite membranes may also improve the water- retention properties of these membranes under low-humidity conditions. We have examined a number of composite polymer/inorganic mem- branes (Nafion/zirconium phosphate, Nafion/tita- nia, Nafion/silica, and Nafion/alumina) in fuel cells at elevated temperatures. Little correlation has been found between the fuel cell performance and chemical formulation, and this has led us to sug- gest that mechanical properties may play an im- portant role in the improved performance of Nafion composite membranes in fuel cells. Because of the evidence for mechanical proper- ties affecting the water content of a polymer mem- brane, a program was initiated to measure the physical and mechanical properties of Nafion and Nafion composite membranes, especially under con- ditions relevant to PEM fuel cell operation (elevated temperature, elevated water activity, and con- strained environments). In this article, we describe a variety of physical, mechanical, and electrical measurements for Nafion and Nafion/titania com- posite membranes. Some are standard measure- ments, such as weight gain, dimensional changes, and tensile testing. Other measurements are less common, including long-term creep, swelling pres- sure, and proton conductivity under load. Several of these new measurement techniques permit us to follow the dynamic changes of polymer ionomers. These measurements give greater appreciation of the complex property changes of the polymer mem- branes in the environment of a fuel cell. EXPERIMENTAL Membrane Preparation Extruded Nafion 115 films (DuPont) were used as the base material against which other membrane formulations were compared. Recast Nafion mem- branes were prepared from a 15 wt % Nafion solu- tion (Liquion 1100, Ion Power) mixed with isopro- pyl alcohol (IPA). The solution was cast onto a flat, glass surface, and the solvent was removed at $70 8C. After the solvent was removed, the mem- branes were annealed at $165 8C for 1 h. To obtain uniform, high-purity films, the membranes were cleaned with a standard treatment procedure: (1) boiling in 3% hydrogen peroxide for 1 h to oxidize organic impurities, (2) rinsing with boiling water for 1 h, (3) boiling in 0.5 M sulfuric acid for 1 h to remove ionic impurities, and (4) rinsing again in boiling water to remove any excess acid. Nafion/TiO2 composite membranes were pre- pared by the recasting of a 15 wt % Nafion solution Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polbPDF Image | Properties of Nafion and Titania Nafion Composite Membranes
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