(188ag) Supercritical Fluid CO2 Processing and Counter Ion Substitution of Nafion Membranes

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 perfluorinated backbone, which make processing challenging and limited by phase equilibria considerations. This study used supercritical fluid (SCF) CO₂ as a processing alternative, since the gas-like mass transport properties of SCFs allow for better penetration into the membranes, and the perfluorinated groups are very CO₂-phillic. The morphology of these membranes was modified with SCF CO₂ at 40°C and 73.8 bars. In addition, the acid sulfonic groups were partially or totally exchanged using six different cations: Al⁺³, Fe⁺³, Ca⁺², Ba⁺², Cu⁺² and K⁺¹, to create highly selective polymer-metal nanocomposite membranes (PMNM). The resulting membranes were characterized using AC impedance spectroscopy (10mHz - 100kHz) to obtain proton conductivities, and thermogravimetric analysis to evaluate their degradation sensitivity. Although in general terms, the incorporation of metal substitution increased the thermal stability of the membranes, its effect on the proton conductivity was unique for each of the cations studied and will be discussed detail.