Abstract
The physical and gas-transport properties of Fe(3+) cross-linked Nafion membranes were examined. Wide-angle X-ray diffraction results revealed a lower crystallinity for Nafion Fe(3+) but showed essentially no changes in the average chain spacing upon cation exchange of Nafion H(+). Raman and Fourier transform infrared spectroscopy techniques qualitatively measured the strength of the ionic bond between the Fe(3+) cations and sulfonate anions. Thermal gravimetric analysis indicated that the incorporation of Fe(3+) adversely affected the thermal stability of Nafion due to the catalytic decomposition of perfluoroalkylether side chains. Gas sorption isotherms of Nafion Fe(3+) measured at 35 °C up to 20 atm exhibited a linear sorption uptake for O(2), N(2), and CH(4) following Henry's law and slight concave behavior for CO(2). Pure-gas permeation results showed reduced gas permeability but higher permselectivities compared to Nafion H(+) with αN(2)/CH(4) = 4.0, αCO(2)/CH(4) = 35, and αHe/CH(4) = 733 attributable to the strong physical cross-linking effect of Fe(3+) that caused chain stiffening with enhanced size-sieving behavior. Gas mixture permeation experiments using 1:1 molar CO(2)/CH(4) feed demonstrated reduced CO(2) plasticization for Nafion Fe(3+). At 10 atm CO(2) partial pressure, CO(2)/CH(4) selectivity decreased to 28 from the pure-gas value of 35, which was a significant improvement compared to the performance of a Nafion H(+) membrane.