Self-Cross-Linked Sulfonated Poly(ether ether ketone) with Pendant Sulfoalkoxy Groups for Proton Exchange Membrane Fuel Cells

Among alternative aromatic proton exchange membranes (PEMs) such as the perfluorosulfonic acid one, side-chain-type polymers have drawn great attention due to the formation of hydrophilic and hydrophobic domains and improvement in proton conductivity. In this work, we report the self-cross-linked sulfonated poly(ether ether ketone) with pendant sulfoalkoxy groups (SPEEK74-O-PAx) prepared by the reaction of hydroxy-functional SPEEK (SPEEK74-OH) with 1,3-propane sultone. The self-cross-linked networks are formed through the electrophilic aromatic substitution reaction catalyzed by protons from the grafted sulfopropyl groups without reducing the sulfonic acid groups. Higher tensile strength, lower water uptake, and in-plane swelling ratio but higher relative water retention than SPEEK74-OH are observed for SPEEK74-O-PAx. The proton conductivity of SPEEK74-O-PA100 is up to 0.1094 S cm–1 at 25 °C, and 0.2003 S cm–1 at 80 °C, which surpasses that of Nafion 212 at each temperature. The introduction of pendant flexible sulfoalkoxy side chains that are favorable for forming distinct steady hydrophilic and hydrophobic domains, and further confirmed by TEM images, can account for the high conductivity. Furthermore, SPEEK74-O-PA100 exhibits the highest power density of 1.358 W cm–2 at a current density of 3.0 A cm–2, which is superior to the performance of Nafion 212 (0.975 W cm–2) and at a high level for aromatic PEMs. The combination of a facile fabrication procedure and remarkable single cell performance enables SPEEK74-O-PAx to be promising membranes for PEM fuel cells.