Synthesis and properties of phosphonated polysulfones for durable high-temperature proton exchange membranes fuel cell

A novel photocatalytic phosphonylation approach was firstly developed to prepare phosphonated poly (phenylene sulfone) (P-PPSU) by post-phosphonylation of brominated poly (phenylene sulfone) (Br-PPSU) and subsequent acidification. The proton conductive P-PPSU polymers are found to be excellent binder materials in catalyst layer to reduce the decay of operating performance of high-temperature proton exchange membrane fuel cell (HT-PEMFC). After irradiation with a LED area source (λ = 365 nm), the P-PPSU polymers with ion-exchange capacity (IEC) values ranging from 1.41 to 2.75 meq/g are obtained successfully with the degree of post-phosphonylation in the range of 18–32%, and without significant degradation of polymer backbone. All the membranes show excellent mechanical properties as well as superior thermal and antioxidant stability. Although the proton conductivity of the P-PPSU-2.75 membrane at high temperature without extra humidification is only 0.30 mS/cm at 160 °C, the peak power density of 242 mW/cm2 is achieved when it is employed as binder in H2/O2 fuel cell operation at 160 °C. This value is lower than that of the fuel cell using PTFE as binder (379 mW/cm2), probably because the PTFE binder in the catalyst layer increases the solubility and diffusivity of oxygen in phosphoric acid (PA) and improves the oxygen reduction kinetics in cathode. However, excellent durability of 200 h is observed in fuel cells operated at 160 °C with P-PPSU-2.75 polymer binder with no significant decrease in performance, while the performance of the fuel cell with PTFE binder decreases rapidly to half of its original value in 150 h. These results suggest that the phosphonated and proton conductive P-PPSU polymer is a promising binder material in catalyst layer for highly durable HT-PEMFCs.