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A recently developed high temperature microelectrophoresis cell was employed for ζ-potential measurements at the rutile/aqueous solution interface over a wide range of pH at temperatures of 25, 120, and 200 °C. Water, 0.001 mol kg -1 NaCl(aq), and 0.01 mol kg -1 NaCl(aq) solutions were tested at these temperatures. The desired pH values were attained by adding either HCl(aq) or NaOH(aq). The obtained experimental data allowed us to estimate isoelectric point pH values (pHiep) of 5.26 ((0.45) at 25 °C, 5.13 ((0.37) at 120 °C, and 4.50 ((0.55) at 200 °C. This decrease in pHiep values with increasing temperature agrees with the decease in point of zero net proton charge (pHznpc) pH values (as determined by potentiometric titration) observed for similar rutile powders in our previous studies. ζ-potential data were combined with the proton charge results and rationalized using a surface complexation modeling approach. Modeling results indicate that ζ-potentials are expressed at a distance that is generally equal to (within error) the so-called diffuse double layer thickness (κ -1 ).

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Conduction and diffusion processes in Nafion 117 polymer electrolyte were analyzed using all-atom molecular dynamics models. The diffusivity of methanol, water, hydronium, and proton in Nafion and the conductivity of Nafion were evaluated in a wide range of temperatures from 20 to 120°C for various water contents ͑ = 3.0, 13.0, and 22.0͒. To validate the simulation results, the conductivity of Nafion was evaluated using a four-probe conductivity measurement apparatus at temperatures from 20 to 120°C in humidified air ͑RHϭ50 to 100%͒ and in liquid water. The permeability of methanol in Nafion was evaluated using a hightemperature membrane diffusion cell. The calculated values and experimental results are generally in good agreement with maximum differences within 50% at room temperature or 100% at higher temperatures. © 2006 The Electrochemical Society. ͓DOI: 10.1149/1.2388735͔ All rights reserved. Nafion ͑DuPont͒, a perfluorosulfonic acid polymer, is the most widely studied polymeric electrolyte for polymer electrolyte fuel cell ͑PEFC͒ applications due to its good mechanical properties, chemical stability, and high ionic conductivity. Nafion membranes are used as molecule separators and proton conductors in membrane electrode assemblies while Nafion ionomers are used in catalyst layers to enable proton access to the catalysts and to promote their electrocatalytic activities. While the transport properties of the bulk Nafion membranes can be readily evaluated experimentally, the transport properties of the ionomer clusters or thin film in the catalyst layers remain ill-defined. Simulation methods allow determination of the transport properties of the microscopic clusters. The proton conductivity of hydrated acidic polymers including Nafion strongly depends on the hydration level or water content that may vary widely depending on the humidification level of the reactant streams, reaction rates, and the location of the polymer in a fuel cell. There is enormous scientific and engineering need for the capability of analyzing the transport processes. Efforts of modeling the hydrated acidic polymer electrolytes are aimed at establishing the relationship between the microstructure of a polymer and the physiochemical and transport properties of the polymer. Because the proton conduction in the hydrated acidic polymers is facilitated by the hydrophilic domains or channels containing proton carriers and water, the proton conduction in the polymers is closely related to the proton conduction in aqueous solutions. An extraordinary feature of the proton conduction in liquid water with excess protons is the anomalous high mobility of protons as compared with those of Na + and K + . This leads to a quest for a detailed rational picture of the so-called "Grotthuss mechanism," "structural diffusion," or proton hopping mechanism that the anomalous high mobility is mainly due to a sequence of proton transfer reactions between water molecules rather than the en masse migration of the proton carriers or "vehicular diffusion." However...

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