(330v) Transport Properties Of Supercritical Fluid Processed Proton Exchange Membranes: Methanol Permeability And Proton Conductivity

Proton exchange membranes (PEMs), commonly used in direct methanol fuel cells (DMFC), are typically limited by either high methanol permeability (also known as the cross-over limitation) or low proton conductivity. These membranes have phase segregated rubbery, glassy and ionic domains, which make processing challenging and limited by phase equilibria considerations. This study proposes to use supercritical fluids (SCFs) processing as an alternative, since the gas-like mass-transport properties of SCFs could allow for better penetration into the membranes and the use of co-solvents could also change the morphology of the PEMs, fine-tuning the physical and transport properties (e.g., fuel permeability and proton conductivity) needed for effective DMFC performance. This investigation focused on the effect of SCF processing on the physical and transport properties of tri-block copolymer ionomers commonly used as proton exchange membranes (PEMs). The PEMs selected included: Nafion®, sulfonated poly-(styrene-isobutylene-styrene) (SIBS) and sulfonated poly-(styrene-ethylene-ran-butylene-styrene) (SEBS). Processing was performed at 200 bars and 40 ºC 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. Proton conductivity was measured using impedance spectroscopy. Measured permeabilities were of the order of 10-8 cm2/s, two orders of magnitude lower than unprocessed membranes for Nafion®. However, changes in proton conductivity were short of the required level (0.1 S/cm) and very sensitive to the processing orientation. These results coupled with thermogravimetric, swelling, and dynamic mechanical analyses have provided a better understanding of the structure property relation of these complex three-dimensional phase-segregated membranes.