New sulfonated poly (ether ether ketone) composite membrane with the spherical bell-typed superabsorbent microspheres: Excellent proton conductivity and water retention properties at low humidity

Good proton conductivity and water retention are essential for proton exchange membranes (PEMs). However, proton conductivity of existing PEMs decreases sharply at low humidity, which seriously restricts the efficient and stable operation of fuel cell system. In this study, we design a spherical bell-typed superabsorbent microsphere with imidazole groups (SBSM) using the distillation-precipitation polymerization method, and incorporate it into sulfonated poly ether ether ketone (SPEEK) matrix to enhance proton conductivity. Benefited from the hygroscopic 3D framework of the superabsorbent core and the special hollow structure of SBSM, the composite membrane remarkably raises the water retention and proton conductivity. Meanwhile, the carboxylic acid, sulfonic acid and Lewis basic imidazole generate two types of acid-base pairs, which serves as proton acceptors and donors to accelerate the formation of low energy paths at the membrane interfaces. The composite membrane with 15 wt% fillers exhibits the highest water retention of 15.72% and the highest proton conductivity of 0.0284 S cm(-1) at room temperature and 20% relative humidity (RH). These improvements are attributed to the unique structure of the SBSMs that provides a stable aqueous environment and additional proton conduction pathways in the membrane.