(315a) Supercritical Fluid Processing of Nafion® Membranes: Methanol Permeability and Proton Conductivity

Nafion® membranes commonly used in direct methanol fuel cells (DMFC), are typically limited by high methanol permeability (also known as the cross-over limitation). These membranes have phase segregated sulfonated ionic domains in a teflon backbone, which make processing challenging and limited by phase equilibria considerations. This study used supercritical fluids (SCFs) as a processing alternative, since the gas-like mass-transport properties of SCFs allow for better penetration into the membranes and the use of polar co-solvents could also influence their morphology, fine-tuning the physical and transport properties. This investigation focused on the effect of SCF processing using SCF CO2 with and without different co-solvents. A variety of co-solvents were used including: 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). Measurements of methanol permeabilities were made using an FT-IR technique, monitoring the methanol concentration with time through the membrane, while proton conductivity was measured using impedance spectroscopy. SCF processing reduced the methanol permeability of Nafion® as much as two orders of magnitude with some co-solvents, while proton conductivity was maintained. In addition, measurements were sensitive to the processing orientation (e.g., normal vs parallel flow). Dynamic mechanical analysis (DMA) revealed an increase in the relaxation around room temperature, which could be an indication of the increased phase segregation after the SCF processing.