(776b) Structure-Property Relationships In Protic Ionic Liquids for High-Temperature PEM Fuel Cells

Contemporary proton exchange membrane (PEM) fuel cells operate at temperatures below 80 °C. However, there are important advantages in operating the fuel cell at a temperature between 120 and 180 °C. These include simpler thermal and water management, and higher tolerance of the platinum catalyst at the anode to carbon monoxide impurity in the reformed fuel gas stream. Polymer electrolyte membranes that rely on water for proton conduction are unsuitable for operation at higher temperatures, because of a large decrease in proton conductivity with a decrease in water activity in the membrane.

In this work, we studied thermophysical and electrochemical properties of several fluorinated and non-fluorinated protic (Brønsted acid–base) ionic liquids based on protonation of the the Brønsted bases, trialkylamines and imidazole, with the Brønsted acids, methanesulfonic acid and perfluoromethanesulfonic acid. It is expected that the structure–property relationships derived from this study will be useful in the design of novel high-temperature composite electrolyte membranes.

Properties such as density, viscosity, self-diffusion coefficient, conductivity, and the temperatures of glass-transition, crystallization, melting, and decomposition were determined, and correlated to the chemical structures of the cation and the anion. The experimental techniques used include cone and plate viscometry for measuring shear-rate and temperature dependent viscosity, electrochemical impedance spectroscopy with electrode equivalent circuit analyses for ionic conductivity determination, and pulsed-field-gradient spin-echo NMR spectroscopy for the measurement of diffusion coefficients of cations and anions. Fluorination of the anion was found to have a strong effect on all the transport properties. It was also found that the protic ionic liquids of the present study differed notably from the non-protic imidazolium iodide ionic liquids that have been recently studied [1]. The order of dependence of molar conductivity on viscosity, or diffusion coefficient, was significantly lower than 1 suggesting deviations from the Nernst-Einstein behavior noted for the non-protic ionic liquids studied previously [1]. A detailed analysis of these results will be presented in this talk.

References

1. Ganapatibhotla, L. V. N. R.; Zheng, J.; Roy, D.; Krishnan, S. PEGylated Imidazolium Ionic Liquid Electrolytes: Thermophysical and Electrochemical Properties. Chem. Mater. 2010, 22, 6347-6360.