The solid electrolyte interface ͑SEI͒ formation on composite graphite and highly oriented pyrolytic graphite in a vinylene carbonate ͑VC͒-containing electrolyte was analyzed using evolved gas analysis, Fourier transform infrared analysis, twodimensional nuclear magnetic resonance, X-ray photoelectron spectroscopy, time of flight-secondary-ion mass spectrometry, and scanning electron microscopy. We found that the SEI layers derived from VC-containing electrolytes consist of polymer species such as poly ͑vinylene carbonate͒ ͑poly͑VC͒͒, an oligomer of VC, a ring-opening polymer of VC, and polyacetylene. Moreover, lithium vinylene dicarbonate, (CHOCO 2 Li) 2 , lithium divinylene dicarbonate, (CHvCHOCO 2 Li) 2 , lithium divinylene dialkoxide, (CHvCHOLi) 2 , and lithium carboxylate, RCOOLi, were formed on graphite as VC reduction products. The presence of VC in the ethylene carbonate ͑EC͒-based electrolyte caused a decrease in the reductive gases of the EC dimethyl carbonate solvent such as C 2 H 4 , CH 4 , and CO. The VC-derived SEI layer was formed at a potential more positive than 1.0 V vs. Li/Li ϩ . Effective SEI formation by reduction of VC progresses before that of EC. The thermal decomposition temperature of the SEI layer derived from VC shifted to a higher temperature compared to that derived from the VC-free electrolytes. We concluded that the thermal stability of the VC-derived SEI layer has a close relation to high-temperature storage characteristics at elevated temperatures.

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Measurements of the mechanical and electrical properties of Nafion and Nafion/titania composite membranes in constrained environments are reported. The elastic and plastic deformation of Nafion-based materials decreases with both the temperature and water content. Nafion/titania composites have slightly higher elastic moduli. The composite membranes exhibit less strain hardening than Nafion. Composite membranes 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 membrane 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 hysteresis 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 constraints, and these constraints can impact fuel cell performance.

Metal-oxide-recast Nafion composite membranes were studied for operation in hydrogen/oxygen protonexchange membrane fuel cells (PEMFC) from 80 to 130 °C and at relative humidities ranging from 75 to 100%. Membranes of nominal 125 µm thickness were prepared by suspending a variety of metal oxide particles (SiO 2 , TiO 2 , Al 2 O 3 , and ZrO 2 ) in solubilized Nafion. The composite membranes were characterized using electrochemical, X-ray scattering, spectroscopic, mechanical, and thermal analysis techniques. Membrane characteristics were compared to fuel cell performance. These studies indicated a specific chemical interaction between polymer sulfonate groups and the metal oxide surface for systems that provide a good elevated-temperature (i.e., fuel-cell operation above 120 °C) performance. Composite systems that incorporate either a TiO 2 or a SiO 2 phase produced superior elevated-temperature, lowhumidity behavior compared to that of a simple Nafion-based fuel cell. Improved temperature tolerance permits the introduction of at least 500 ppm CO contaminant in the H 2 fuel stream without cell failure, in contrast to standard Nafion-based cells, which fail below 50 ppm of carbon monoxide.