Multiple Hydrogen Bond Systems Boosting High Proton Conductivity of the Comb-Shaped Sulfonated Poly(ether ether ketone) Proton Exchange Membranes

Abstract

Grafting imidazole and sulfonic acid groups onto the side chains of sulfonated poly(ether ether ketone) (SPEEK) is essential to increasing its proton conductivity. By creating double or triple hydrogen bond networks that promote proton transport, two different comb-structured SPEEK proton exchange membranes were synthesized. Herein, SPEEK membranes grafted with an imidazole group (CS-a) and a sulfonic acid group (CSC) have been successfully prepared. More specifically, hydrogen bonds are formed as a result of the acid–base interactions that occur between the amine and sulfonic acid groups and imidazole and sulfonic acid groups. Furthermore, sulfonic acid groups create hydrogen bonds with water when they are moved through water, creating a multiple hydrogen bond system that aids in proton transport. Meanwhile, Fourier transform infrared spectroscopy, X-ray diffraction, and differential scanning calorimetry (DSC) provide additional confirmation of hydrogen bond formation. The material and electrochemical evaluation revealed that although the imidazole group in CS-a promotes proton transport (σ = 58.33 mS/cm), the membrane’s dimensional stability was poor due to its hydrophilicity. The proton exchange membrane (CSC) prepared by grafting sulfanilic acid on the basis of the CS-a structure has excellent proton conductivity (σ = 100.04 mS/cm), which is 138% higher than that of SPEEK. More importantly, the CSC membrane exhibits exceptional dimensional stability. This work suggests a viable process for creating a high-efficiency PEMFC membrane, suggesting that its performance could be improved.