Synthesis of highly sulfonated 2,6-diphenyl-p-Phenylene oxide: A highly conductive membrane

Highly sulfonated aromatic polymers are desired for the possible highest performance of polymer electrolytes in fuel cells and electrolyzer applications. Here, we reported the method to prepare highly sulfonated 2,6-diphenyl-p-phenylene oxide (SP₃O), a commercially available aromatic polymer, with the highest IEC of 4.42 meq/g. Three sulfonation reagents including chlorosulfonic acid (HSO₃Cl), concentrated sulfuric acid (conc. H₂SO₄), and trimethylsilyl chlorosulfonate (TMSCS) were investigated for single and two-step sulfonation reactions. In addition, DMSO-assisted thermal crosslinking method was applied to prepare crosslinked SP₃O membranes to improve their mechanical strength and film-forming ability. Proton exchange membrane from crosslinked SP₃O displayed a proton conductivity as high as 250 mS/cm at 95 % RH and 60° C.