Enhanced water retention and low-humidity proton conductivity of sulfonated poly(ether ether ketone) hybrid membrane by incorporating ellipsoidal microcapsules

Two kinds of polymeric ellipsoidal microcapsules (EMCs) with different functional groups (phosphoric acid groups and imidazole groups) were synthesized via precipitation polymerization and incorporated into sulfonated poly(ether ether ketone) (SPEEK) matrix to prepare hybrid membranes. The structure, thermal stability and composition of EMCs were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). The quasi-one dimensional structure endowed EMCs with high water uptake and water retention. Especially, for the hybrid membranes filled with 5 wt.% phosphoric acid-functionalized polymeric EMCs, water uptake was increased from 19.0% for pristine SPEEK membranes to 58.2%, and the water retention was 15.1% after 180 min testing at 40 °C and 20% RH, which is threefold and ninefold higher than those of pristine SPEEK membranes, respectively. Particularly, the proton conductivity at low RH was still up to 3.51 × 10-3 S/cm after 60 min testing. The results manifested that the ellipsoidal microcapsules, as a new kind of filler, had great potential in enhancing the water retention and low-humidity proton conductivity of proton exchange membranes.