Sulfonated poly(ether ether ketone)/TiO2 double-deck membrane for vanadium redox flow battery application

Abstract A novel sulfonated poly(ether ether ketone) (SPEEK)/TiO 2 double-deck membrane which consists of a layer of SPEEK and a layer of TiO 2 was prepared and investigated for vanadium redox flow battery (VRB) application for the first time. The physicochemical properties of the SPEEK/TiO 2 membrane, including the water uptake, swelling ratio, ion exchange capacity, proton conductivity, VO 2 + permeability and ion selectivity are evaluated in detail, compared to the pristine SPEEK membrane and Nafion 117 membrane. The scanning electron microscopy images reveal its double-deck structure and the structural stability with no delamination, and thermogravimetric analysis (TG) identifies its thermal stability. Among all membranes, the SPEEK/TiO 2 double-deck membrane possesses lowest vanadium ion permeability (6.66 × 10 − 7  cm 2  min − 1 ) and highest ion selectivity (9.46 × 10 4  S min cm − 3 ). The VRB single cell with SPEEK/TiO 2 double-deck membrane shows higher coulombic efficiency (97.0% vs 93.3%) and energy efficiency (85.8% vs 83.7%) compared to that with Nafion 117 membrane at 60 mA cm − 2 . Furthermore, the SPEEK/TiO 2 double-deck membrane exhibits highly stable cell performance after 60 times of cycling tests at 60 mA cm − 2 and lower capacity decay rate than that of Nafion 117 membrane. Therefore, the SPEEK/TiO 2 double-deck membrane exhibits good potential usage in VRB systems.