Morphology and Conductivity of Perfluorinated-Sulfonated Membranes Processed with Supercritical Fluids

This investigation studies the effect of supercritical fluid (SCF) processing on the physical and transport properties of tri-block copolymer ionomers commonly used as proton exchange membranes (PEMs). The hypothesis of the investigation is that the SCF processing will change the morphology of the polymers, increasing the proton conductivity, while reducing the fuel permeability of various PEMs. The PEMs selected include: Nafion®, sulfonated poly-(styrene-isobutylene-styrene) (SIBS), and sulfonated poly-(styrene-ethylene-ran-butylene-styrene) (SEBS). The SCF processing was performed at 200 bar and 40ºC using SCF CO₂ with and without co-solvents. The co-solvents studied include: acetic acid, acetone, acetonitrile, cyclohexanone, ethanol, isopropanol, methanol, methylene chloride, and tetrahydrofuran. They were selected based on their: size, polarity, and chemical nature (e.g., protic or aprotic).

The processed membranes were first characterized using density to verify preservation of the integrity of the membranes. The second test performed was the ion-exchange capacity (IEC); a measure of the sulfonic groups available for proton conductivity. The final step of the investigation was the study of the methanol permeability and the kinetics of the proton conductivity for the processed membranes.

The outcome from this investigation can significantly improve the performance of PEMs for fuel cell applications, but it can also be significant in chemical protective clothing and the design of chemical sensors.