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TITANIA/NAFION COMPOSITE MEMBRANES 2333 Table 1. Physical Characteristics of Nafion and Nafion/Titania Composite Membranes Wet Density (g/cm3) Membrane (g/cm3) 23 8C 100 8C IEC (lequiv/g) Water Swelling (DL/L) 23 8C 100 8C Resistivity at 23 8C and 100% RH (O cm) Dry Density Nafion115 1.9260.08 1.6560.08 1.4260.08 950625 0.1060.01 0.2060.01 12.460.5 (extruded) Nafion115 1.9660.10 1.6860.08 1.4460.08 960625 0.1260.01 0.2460.01 10.660.5 (recast) Nafion/titania 1.94 6 0.11 1.68 6 0.08 1.50 6 0.08 115(3wt%) After being weighed, the membranes that had been removed from boiling water were placed in beakers of water at room temperature for 24 h and then reweighed. The mass of the membranes was the same as that determined immediately after their removal from water at 100 8C. This simple experiment demonstrates that water sorption by Nafion-type membranes can be controlled by kinetics; water sorption is not thermodynamically equilibrated within 24 h at 25 8C, and substantial hysteresis can exist, depending on the history of the membrane. There was no measurable difference in the swel- ling behavior or IEC of Nafion and Nafion/TiO2 membranes. The resistivity of the membranes measured ex situ is about a factor of 2 less than the resistivity inferred from the ohmic region of the iv curve in the fuel cell. Part of the difference in the resistivity is due to using the nominal thickness of the membrane (127 lm) to determine the resistiv- ity in the fuel cell. The membrane thickness increases from water sorption. The transverse re- sistivity (across the membrane as it would be mea- sured in the fuel cell) scales with the membrane thickness. The longitudinal resistivity (measured in the ex situ device) scales inversely with the thickness. If the membrane thickness is swollen by 25%, the in situ fuel cell resistivity is in- creased by 25%, whereas the ex situ resistivity is reduced by 25%. Making these corrections brings the values in Table 1 into close agreement with the resistivity determined from the slope of the iv curve shown in Figure 4. Mechanical Properties Tensile Tests Water sorption is accompanied by membrane swel- ling, which depends on the mechanical properties of the polymer or composite. We have looked at Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb 970 6 25 0.12 6 0.01 0.23 6 0.01 9.3 6 0.5 several different mechanical property measure- ments of the membrane materials. The most com- mon polymer tests are tensile tests. A typical ten- sile test for extruded Nafion and a Nafion/titania composite membrane is shown in Figure 5. A vari- ety of values can be extracted from the tensile test- ing; we focus on two properties, the elastic modu- lus (the slope of the stress–strain curve at small strains) and the plastic modulus (the slope of the stress–strain curve beyond the yield point). A true plastic would show no increase in stress with increasing strain above the yield point. Nafion showed increasing stress as it was strained past the yield point. A positive slope of the stress–strain curve past the yield point is indicative of strain hardening.41,42 Figures 6 and 7 summarize our stress–strain measurements for Nafion and Nafion/titania com- posite membranes. As expected, Figure 6 shows that the elastic modulus decreases with increasing temperature. At room temperature, the elastic Stress–strain response of extruded Nafion and Nafion/titania composite membranes. The strain rate was 5 cm/min. The data were obtained at room tem- perature (22–25 8C). The water content for each mem- brane was k $ 10.9 H2O/SO3. Figure 5.PDF Image | Properties of Nafion and Titania Nafion Composite Membranes
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