Studies on the SPEEK membrane with low degree of sulfonation as a stable proton exchange membrane for fuel cell applications

Sulfonated poly (ether ether ketone) (SPEEK) with a low degree of sulfonation (DS = 40%) was prepared for proton exchange membrane fuel cells (PEMFC). Poly (ether ether ketone) (PEEK) was sulfonated in concentrated H2SO4 under N2 atmosphere and characterized by the hydrogen nuclear magnetic resonance (H-NMR) technique. After preparation of the SPEEK polymer, the obtained polymer was dissolved in dimethylacetamide (DMAC) solvent and then the solution casting method was applied for the fabrication of membranes. Water uptake behavior, ion exchange capacity (IEC), and the proton conductivity at different temperatures and different relative humidity were determined and compared with a commercial Nafion 117 membrane. The IEC and the proton conductivity results showed that the sulfonation process successfully created   proton conduction channels. In addition, the thermal, mechanical, chemical, and hydrolytic stabilities were thoroughly investigated for the prepared membrane. The low degree of sulfonation SPEEK showed a desirable chemical durability with a higher dimensional, mechanical, and thermal stability than Nafion-117 but lower water uptake and proton conductivity.