Sulfonated poly(fluorenyl ether ketone nitrile) electrolyte membrane with high proton conductivity and low water uptake

High molecular weight sulfonated poly(fluorenyl ether ketone nitrile)s with different equivalent weight (EW) from 681 to 369 g mequiv.−1 are synthesized by the nucleophilic substitution polycondensation of various amounts of sulfonated difluorobenzophenone (SDFBP) and 2,6-difluorobenzonitrile (DFBN) with bisphenol fluorene (BPF). The synthesized copolymers are characterized by 1H NMR, FT-IR, TGA, and DSC techniques. The membranes cast from the corresponding copolymers exhibit superior thermal stability, good oxidative stability and high proton conductivity, but low water uptake due to the strong nitrile dipole interchain interactions that combine to limit swelling. Among all the membranes, the membrane with EW of 441 g mequiv.−1 shows optimum properties of both high proton conductivity of 41.9 mS cm−1 and low water uptake of 42.6%. Accordingly, That membrane is fabricated into a membrane electrode assembly (MEA) and evaluated in a single proton exchange membrane fuel cell (PEMFC). The experimental results indicate its similar cell performance as that of Nafion® 117 at 70 °C, but much better cell performance at higher temperatures. At the potential of 0.6 V, the current density of fuel cell using the prepared membrane and Nafion® 117 is 0.46 and 0.25 A cm−2, respectively. The highest current density of the former reaches as high as 1.25 A cm−2.