Optimal thermal treatment conditions for durability improvement of highly sulfonated poly(ether ether ketone) membrane for polymer electrolyte fuel cell applications

Abstract In this study, a series of sulfonated poly(ether ether ketone) (SPEEK) membranes are prepared and thermally treated in different conditions, i.e., temperature (120–180 °C), time (6–42 h), and residual solvent (RS) values (15–45%). Proton conductivity, water uptake, swelling ratio, hydrogen permeability, mechanical properties, and chemical stability of samples are systematically investigated. Also, membranes' characteristics are studied using FTIR, TGA, and AFM. Two stages of “annealing dominated” and “cross-linking dominated” were observed during thermal treatment. Annealing dominated samples exhibited higher flexibility and overall stability with annealing-induced homogenous microstructure while cross-linking dominated samples showed poor mechanical properties and proton conductivity. Results indicate that the treatment of SPEEK membranes with higher RS value facilitates cross-linking at much lower temperatures with a more controlled rate to inhibit excessive cross-linking. Furthermore, response surface methodology suggested that prolonged thermal treatment at low temperatures (