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Figure 10. Mass gain of Nafion and Nafion/titania composite membranes from satu- rated water vapor at different temperatures. because the simple analysis misses key physics associated with the water sorption process. The drying temperature before absorption, 130 8C for 1 h, is higher than those used by other research- ers, who dried Nafion below its Tg of 110 8C to avoid disturbing its microstructure.39,47 How- ever, drying at 130 8C more closely matches the conditions at which an MEA is hot-pressed, and this is relevant to the changes encountered dur- ing water absorption in a fuel cell. The effect of the thermal history on membrane water absorp- tion is an important question, but a complete study is beyond the scope of this article. The dynamics of water absorption were also seen in the changing membrane resistance as it absorbed water inside the device depicted in Fig- ure 2. A typical result is shown in Figure 11 for a membrane under a compressive load of 40 kPa at 21 8C. Figure 11 shows that water was rapidly absorbed into the membrane, reducing the re- sistivity by over an order of magnitude in the first 100 s. The rate of change of the resistivity slowed dramatically after the initial decrease. The membrane resistivity kept decreasing for $100,000 s. The membrane resistivity decreased from 60 to 15 O cm between 1000 s and 5000 s and to less than 13 O cm after 20,000 s. It reached its equilibrated resistance of 12.6 O cm after 40,000 s. The resistivity decreased slightly Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb faster for a Nafion/titania (3 wt %) 115 mem- brane than extruded Nafion 115 at room temper- ature; this is consistent with the water uptake measured by the change in mass. Preliminary experiments have shown that the resistivity change is slowed by an increasing applied load, but we do not yet have sufficient data to quantify these phenomena. Membrane Proton Conductivity A polymer membrane in a fuel cell is in a con- strained environment under compression. Figure 12 shows the resistivity as a function of the applied stress on the membrane for both extruded Nafion 115 and Nafion/3 wt % TiO2 115 membranes. The resistivity was based on the dry membrane thick- ness. The stress was increased from 0 to 7.25 MPa in incremental steps every minute. After 1 min at each applied stress, the resistivity change was slow (<2%/min), but we did not wait for full equili- bration. The membrane resistivity had increased by $15% at the maximum applied stress of 7.25 MPa. After the maximum was reached, the stress was reduced with the reverse sequence. There was a substantial hysteresis in the resist- ance at the same stress level on the return path. We did find that the resistivity at $0 stress was recovered but only after $1000 s. This hysteresis of the resistance with stress was reproducible. TITANIA/NAFION COMPOSITE MEMBRANES 2337PDF Image | Properties of Nafion and Titania Nafion Composite Membranes
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