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

A novel sulfonated poly(ether ether ketone) (SPEEK)/TiO2 double-deck membrane which consists of a layer of SPEEK and a layer of TiO2 was prepared and investigated for vanadium redox flow battery (VRB) application for the first time. The physicochemical properties of the SPEEK/TiO2 membrane, including the water uptake, swelling ratio, ion exchange capacity, proton conductivity, VO2 + 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/TiO2 double-deck membrane possesses lowest vanadium ion permeability (6.66 × 10− 7 cm2 min− 1) and highest ion selectivity (9.46 × 104 S min cm− 3). The VRB single cell with SPEEK/TiO2 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/TiO2 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/TiO2 double-deck membrane exhibits good potential usage in VRB systems.