Perfluorosulfonic acid membranes treated by supercritical carbon dioxide method for direct methanol fuel cell application

Perfluorosulfonic acid (PFSA) membranes were treated by supercritical carbon dioxide (Sc-CO2) method for direct methanol fuel cell application. After the treatment in Sc-CO2 at a desired temperature, the microstructure of the PFSA membranes was changed greatly. Small-angle X-ray scattering measurement indicated that a long-range order developed and the size of the ion cluster in the fully hydrated membranes became much smaller. Wide-angle X-ray diffraction measurement showed that the relative crystallinity of the PFSA membranes increased due to Sc-CO2-induced crystallization. The macro-physical performances of the membranes were also investigated: swelling ratio of the membrane became much smaller; solubility of the membrane in ethanol/water solvent decreased dramatically; density of the wet membrane increased evidently. Differential scanning calorimetry measurement indicated that the nonfreezable water per volume of the fully hydrated membranes increased notably. As a result, the methanol permeability of the PFSA membranes was significantly reduced without the sacrifice of proton conductivity, and the mechanical properties increased evidently. All the results indicated that this novel treatment in Sc-CO2 atmosphere is a promising method to improve the properties of membrane for direct methanol fuel cell application.