Structural, Thermal, and Gas-Transport Properties of Fe3+ Ion-Exchanged Nafion Membranes

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.