Synthesis and properties of sulfonated fluorene-based poly(phenyl ketone) for highly stable proton exchange membranes

Polymer with fluorene-based units has garnered significant attention and has been shown an effective way to improve the performance of proton exchange membranes due to its inherent rigidity and multiple sulfonation sites. Nevertheless, fluorene-based sulfonated poly(ether ketone)s exhibited inadequate oxidative stability resulting from the heteroatoms in the main chain. Herein, a series of poly(phenyl ketone)s polymers (PPK-DSF-x) without heteroatoms in the main chain were constructed by Yamamoto coupling and hydrolysis reactions. Since robust poly(phenyl ketone) aromatic backbone ensured commendable chemical and dimensional stability, and sulfonated fluorene-based units promoted the formation of hydrophilic and hydrophobic microscopic phase separation, the PPK-DSF-x membranes exhibited superior stability and proton conductivity. Remarkably, PPK-DSF-45 showed excellent proton conductivity at 80 °C (137.5 mS cm-1). In addition, the oxidative stability of the poly(phenyl ketone) PEM was also excellent, maintaining its integrity after immersion in Fenton's reagent for 270 min. Finally, we found that the PPK-DSF-45 membrane achieved a power density of 270 mW cm-2 in the fuel cell, surpassing Nafion 212 (245 mW cm⁻2). These results indicated the potential application of fluorene-based poly(phenyl ketone)s PEMs in PEMFCs.