Transport Properties of Sulfonated Poly(ether ether ketone) Membranes with Counter-Ion Substitution

Transport properties of sulfonated poly(ether ether ketone) (SPEEK) were analyzed as a function of counter-ion substitution (e.g., Ba²⁺, Ca²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Cu²⁺, Zn²⁺, K⁺) for direct methanol fuel cell (DMFC) applications. The polymeric membranes were characterized using several materials characterization techniques including: elemental analysis (EA), thermogravimetric analysis (TGA), water swelling, Fourier-transform infrared spectroscopy (FT-IR), atomic force microscopy (AFM) and small angle x-ray scattering (SAXS). These results were used to understand the changes in the physical, chemical, and thermal properties upon counter-ion substitution and to help explain the DMFC transport property results (e.g., proton conductivity and methanol permeability). Significant differences in their thermal, physical and transport properties were observed for SPEEK when exchanged with the different counter-ions. Alternative transport mechanisms generated by the location of the counter-ions in the different ionic domains and the role of water is discussed. This led to normalized selectivities (proton conductivity over methanol permeability divided by the respective values of Nafion®) better than Nafion®, but with significant differences between the counter-ions studied, ranging from 3.8 (SPEEK-Mg²⁺) to 36.7 (SPEEK-Ca²⁺).