(510s) Highly Conductive Ionic Liquid-Polymer Membranes

Polyelectrolyte membranes are of great interest due to their application in membrane-based separations, fuel cells, sensors, and actuators, where high ionic transport is desired for process efficacy. However, ionic conductivity is usually a strong function of membrane water content. Therefore, high temperature operation results in membrane dehydration and consequently severe performance losses. This project aims to replace water as the medium for ionic transport with a class of organic salts commonly known as ionic liquids (ILs). In particular, room temperature ionic liquids are of interest due to their negligible vapor pressures, high degradation temperatures, large electrochemical windows, and high ionic conductivities. Successful implementation of ionic liquids as a substitute for water will allow for water-free operation of current polyelectrolyte membranes, expanding their range of operating conditions and utility.

In this work, PVA (poly(vinyl-alcohol)) and Nafion^® membranes with varying amounts of the ionic liquid, 1-ethyl-3-methylimidazolium dicyanamide [EMIM][N(CN)2]), were prepared and characterized. The PVA-IL membranes were solution cast, while Nafion^®-IL membranes were prepared via sorption. The conductivity results show that with increasing IL content in the PVA membrane, the room temperature conductivity increases seven orders of magnitude and asymptotically approaches the ionic conductivity of the pure IL. High temperature, low humidity conductivity was measured and showed a maximum of 68 mS/cm at 120^oC and 10%RH. This result compares favorably with Nafion^®, which was measured to have a conductivity of 1 mS/cm under similar conditions and 145 mS/cm at 80^oC and 90%RH. The Nafion^®-IL system showed promising results with higher conductivities at low humidities, but lower conductivities at high humidities compared to that of pure Nafion^®. Tensile tests on the IL-polymer membranes show that the ionic liquid acts as a plasticizer - modulus decreases, while the elongation to break increases with increasing IL. Thermogravimetric analysis shows the representative IL-polymer membranes are stable above 200^oC and scanning electron micographs reveal phase separation between the IL and polymer. These results provide insights into the interactions between ionic liquid and the host polymer, which may aid in the design of new polyelectrolyte membranes.