Synthesis and characterization of sulfonated poly(arylene ether ketone/ketone phosphine oxide)s as proton exchange membranes

Sulfonated poly(arylene ether ketone phosphine oxide)s (sPEKP) and sulfonated poly(arylene ether ketone ketone phosphine oxide)s (sPEKKP) were prepared by polycondensation for the first time. The phosphine oxide moieties in products were expected to enhance water retention and improve the overall properties. Their properties as proton exchange membranes were investigated and compared. They all showed high thermal stability. Moreover, the sPEKP membranes showed low swelling though they have considerable water uptake at 80 °C. However, the sPEKKP membranes displayed much higher water uptake and swelling than the sPEKP membranes with an equivalent ion exchange capacity (IEC) at elevated temperatures, which is due to the structural difference between them. More importantly, the sPEKP and sPEKKP membranes indicated outstanding oxidative stability because of the passivation effect of phosphine oxide groups. In short, the sPEKP membranes showed good overall properties. For example, sPEKP-400 not only showed high thermal and oxidative stability, but also displayed a swelling of 21% and a proton conductivity of 0.075 S/cm at 80 °C.