Copolymerized sulfonated poly(oxindole biphenylene) polymer electrolyte for proton exchange membrane fuel cells

The ether-free aromatic polymer electrolytes have the potential to be used as proton exchange membranes (PEMs) and binders materials in fuel cells. Nevertheless, it is still a technical challenge to improve their mechanical and chemical stability. Here, we prepared a series of quantitatively sulfonated poly(oxindole biphenylene) polymers with common substituent groups (SPOBPn-RPOBP(100-n), R=F, Cl, Br and NO2) by simply copolymerization. Substituent groups occupy highly active sulfonation sites of polymer, so the sulfonation degree of the polymer is greatly reduced. They show limited water uptake (33–35 wt%) and swelling ratio (10–13 %) compared with fully sulfonated SPOBP100 (water uptake of 366 wt% and swelling ratio of 78 %) at 80 °C. Thus, the voltage of single-cell assemblyed with SPOBP50-FPOBP50 membrane remains stable for 90 h at 0.3 A/cm2, much higher than that of SPOBP100 (20 h). Moreover, the cells exhibit similar lifetime trends using it as binder materials in catalyst layer. These results indicate that it is possible to prepare electrolyte with controllable sulfonation degree and high stability by post-sulfonation on copolymers containing substituent groups.