Comparison of homogeneously and heterogeneously sulfonated polyetheretherketone membranes in preparation, properties and cell performance

For application in direct methanol fuel cells, polyetheretherketone (PEEK) membranes with different degrees of sulfonation (40-80%) are prepared by both homogeneous and heterogeneous methods. Sulfonation and its degree are identified by means of Fourier transform infrared (FT-IR) spectroscopy and a back-titration method, respectively. Thermal analysis shows that an increase in the sulfonation degree increases the glass-transition temperature and enhances the thermal stability. The room-temperature ion conductivity of the homogeneously sulfonated PEEK (sPEEK) membrane with a 68% degree of sulfonation is higher than that of Nafion^® 117, while its methanol permeability is lower. The tensile strength of the sPEEK membrane is comparable with that of Nafion^® 117 at the equilibrium water swollen state. Various properties of sPEEK membranes prepared by two methods are compared. Overall, the ion conductivity of the homogeneous system is higher than that of the heterogeneous counterpart, but there is little difference in methanol permeability. The cell performance of the homogeneously sulfonated PEEK (homo-sPEEK) membrane is much better than that of the heterogeneously sulfonated PEEK (hetero-sPEEK) and Naifion^® 117 membranes.