PTFE/SPEEK/PDDA/PSS composite membrane for vanadium redox flow battery application

How to solve the crossover of vanadium ions through ion exchange membrane is a key issue in vanadium redox flow battery (VRB), especially for ultra-thin membranes used for VRB to obtain a lower cell resistance. Herein, an ultra-thin (~ 30 μm) PTFE/SPEEK [polytetrafluoroethylene/sulfonated poly(ether ether ketone), P/S] membrane is successfully prepared and modified by using layer-by-layer (LBL) self-assembly technique with polycation poly(diallyldimethylammonium chloride) (PDDA) and polyanion poly(sodium styrene sulfonate) (PSS). P/S membranes are alternatively immersed in positively and negatively charged polyelectrolyte to form 2 to 8 bilayers onto its surface. Consequently, a series of P/S-[PDDA/PSS] _n (n is the number of multilayers) membranes are fabricated. Both the physicochemical properties and VRB performances of the P/S-[PDDA/PSS] _n membranes are then investigated in detail. Results show that the ion selectivity of the P/S-[PDDA/PSS] _n membranes is much higher than that of pristine P/S membrane, especially for P/S-[PDDA/PSS]₆ membrane. As a result, the VRB with the P/S-[PDDA/PSS]₆ membrane exhibits the highest coulombic efficiency (CE) of 96.5% at 80 mA cm⁻², the highest voltage efficiency of 94.7% at 40 mA cm⁻² and the highest energy efficiency of 87.7% at both 40 and 50 mA cm⁻², respectively. In addition, 80 times charge–discharge test proves that the P/S-[PDDA/PSS]₆ membrane possesses high stability and no obvious CE decay after running. All the results show that the LBL technique is an effective way to prepare ultra-thin membrane with high ion selectivity for VRB application.