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Text from PDF Page: 001Mechanical Properties of Nafion and Titania/Nafion Composite Membranes for Polymer Electrolyte Membrane Fuel Cells M. BARCLAY SATTERFIELD,1 PAUL W. MAJSZTRIK,2 HITOSHI OTA,2 JAY B. BENZIGER,1 ANDREW B. BOCARSLY2 1Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544 2Chemistry Department, Princeton University, Princeton, New Jersey 08544 Received 23 January 2006; revised 21 March 2006; accepted 13 April 2006 DOI: 10.1002/polb.20857 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Measurements of the mechanical and electrical properties of Nafion and Nafion/titania composite membranes in constrained environments are reported. The elas- tic and plastic deformation of Nafion-based materials decreases with both the tempera- ture and water content. Nafion/titania composites have slightly higher elastic moduli. The composite membranes exhibit less strain hardening than Nafion. Composite mem- branes also show a reduction in the long-time creep of $40% in comparison with Nafion. Water uptake is faster in Nafion membranes recast from solution in comparison with extruded Nafion. The addition of 3–20 wt % titania particles has minimal effect on the rate of water uptake. Water sorption by Nafion membranes generates a swelling pressure of $0.55 MPa in 125-lm membranes. The resistivity of Nafion increases when the mem- brane is placed under a load. At 23 8C and 100% relative humidity, the resistivity of Nafion increases by $15% under an applied stress of 7.5 MPa. There is a substantial hy- steresis in the membrane resistivity as a function of the applied stress depending on whether the pressure is increasing or decreasing. The results demonstrate how the dynamics of water uptake and loss from membranes are dependent on physical con- straints, and these constraints can impact fuel cell performance. VC 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2327–2345, 2006 Keywords: ionomer; mechanical properties; Nafion; PEM fuel cells; polymer compo- sites; structure-property relations; water sorption INTRODUCTION Polymer electrolyte membrane (PEM) fuel cells based on perfluorinated membranes have success- fully been operated in a temperature range of approximately 50–90 8C.1–3 Efforts to develop poly- mer membranes able to operate above 120 8C have been prompted by the additional benefits of enhanced carbon monoxide (CO) tolerance and improved heat removal.4–8 The most significant Correspondence to: J. B. Benziger (E-mail: benziger@princeton. edu) Journal of Polymer Science: Part B: Polymer Physics, Vol. 44, 2327–2345 (2006) VC 2006 Wiley Periodicals, Inc. barrier to running a polymer electrolyte fuel cell at elevated temperatures is maintaining the proton conductivity of the membrane. Most polymer membranes rely on absorbed water to ionize acid groups and permit proton transport. The conduc- tivity of a dry membrane is several orders of mag- nitude lower than that of a fully saturated mem- brane; proton conductivity increases exponentially with water activity in the membrane. Increasing the fuel cell temperature raises the vapor pressure required to keep a given amount of water in the membrane, thereby increasing the likelihood that water loss will occur and significantly reduce pro- ton conductivity. 2327
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