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Figure 13. Resistivity change during the stress relaxation of Nafion and Nafion/TiO2 composite membranes. on the direction of approach to the applied stress. The stress relaxation hysteresis shown in Figure 13 persisted for more than 200 min with both membranes. Both membranes relaxed back to their zero stress resistivity in approximately 10 min if the applied stress was completely removed. Dynamics of Membrane Swelling Membrane swelling associated with water sorption requires the membrane to develop sufficient inter- nal pressure to overcome the applied stress com- pressing the membrane. The energy of water sorp- tion per unit of volume gives rise to a swelling pressure of the membrane, as measured by the de- vice depicted in Figure 3. Typical data are shown in Figure 14(A,B). Both extruded Nafion 115 and Nafion/3 wt % TiO2 115 membranes generated a swelling pressure of 0.55 6 0.03 MPa for water sorption at 60–90 8C. The swelling pressure changed by less than 10% between 30 and 90 8C. Figure 14(B) is a blowup of the force generated when water is injected and shows that the water sorption and swelling pressure build up quickly within $100 s. After the initial increase in force, there is a slow decrease in the force with a much longer response time, $10,000 s. The slow relaxa- tion to the swelling pressure occurs with a time response similar to that of membrane creep. From limited data for composite membranes, the swel- ling pressure of the Nafion/3 wt %TiO2 composite membranes is the same as that of Nafion within the experimental error. Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb DISCUSSION The studies reported here were initiated to eluci- date how polymer electrolytes in constrained envi- ronments respond to applied stresses. The mem- brane in a PEM fuel cell is constrained between the porous electrodes; the compression sealing of the fuel cell assembly puts the membranes under stress. As the polymer electrolyte absorbs and desorbs water, it swells and shrinks, altering the stress levels and proton conductivity. There is pre- cious little data available concerning the mechani- cal properties of polymer electrolytes as functions of both the temperature and water activity, and there is virtually no data for the properties of these materials in confined environments. The data pre- sented here are by no means complete, but they begin to elucidate how water and temperature affect the mechanical and electrical properties of polymer electrolyte membranes, which may impact fuel cell performance. The addition of TiO2 particles to a Nafion mem- brane improves the fuel cell performance. These improvements have been documented by us and others.36 There have been several different ex- planations for this improvement; most of the theories have focused on greater water sorption in the composite membranes, particularly under reduced-humidity conditions.5,31,48 The fuel cell data shown in Figure 4 were obtained under fully humidified conditions and show that the composite membrane had lower resistivity even at full hu- TITANIA/NAFION COMPOSITE MEMBRANES 2339PDF Image | Properties of Nafion and Titania Nafion Composite Membranes
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