Highly selective sulfonated poly(ether ether ketone)/titanium oxide composite membranes for vanadium redox flow batteries

Abstract To lower the cost and enhance the selectivity of membranes used in vanadium redox flow batteries (VRFBs), sulfonated poly(ether ether ketone) (SPEEK)/ titanium oxide (TiO 2 ) composite membranes with different degrees of sulfonation (DS) (prepared at different time durations of 3 h, 5 h and 7 h) and various TiO 2 nanoparticles loadings (3%, 5% and 7%) were prepared. Optimum DS and TiO 2 nanoparticles loading were confirmed by different characterization techniques including 1 H NMR, scanning electron microscopy, water uptake, swelling ratio, proton conductivity, vanadium ion permeability and thermal stability. Results show that 5 h SPEEK/5% TiO 2 composite membrane possessed comprehensive merits, especially high ion selectivity (72.55 × 10 3  S min cm −3 ) due to the blocking effect of TiO 2 nanoparticles and good proton conductivity. TiO 2 nanoparticles combined very well with SPEEK due to strong interaction, as deduced from flux tests. The chemical and oxidative stability were sufficiently good for applications of VRFB. The VRFB single cell with 5 h SPEEK/5% TiO 2 composite membrane exhibited excellent performance including high coulombic efficiency (98.3%), high energy efficiency (82.9%), long self-discharge time and low capacity loss compared to Nafion 117. Hence, SPEEK/TiO 2 composite membranes can potentially be promising substitute for Nafion in VRFBs.