Building bridges: Crosslinking of sulfonated aromatic polymers—A review

Sulfonated aromatic polymers (SAP) are promising inexpensive polymer electrolyte membranes for fuel cells, but there still exist some challenges about this kind of materials including excessive swelling in water, poor mechanical strength, low dimensional stability, especially for highly sulfonated SAP. Crosslinking is widely proposed as an efficient strategy to deal with these challenges. Here, the state-of-the-art of crosslinked SAP is reviewed and some prospects for further development are outlined. The following points are addressed: (i) Crosslinked SAP via covalent bonds, including esterification, addition, Friedel–Crafts reactions and formation of –SO2– bridges; (ii) crosslinked SAP by ionic bonds; (iii) combined covalent and ionic crosslinking.