Layer-by-layer self-assembly of PDDA/PSS-SPFEK composite membrane with low vanadium permeability for vanadium redox flow battery

Sulfonated poly(fluorenyl ether ketone) (SPFEK) membranes have been first modified by layer-by-layer (LbL) self-assembly of positively charged polyelectrolyte PDDA (poly(diallyldimethylammonium chloride)) and negatively charged PSS (poly(sodium styrene sulfonate)). The membranes were investigated as an ion exchange membrane for vanadium redox flow batteries (VRBs). The permeability of the vanadium ions in VRBs was effectively suppressed by depositing the LbL thin film on the SPFEK membrane. The permeability decreased with increasing the number of PDDA/PSS bilayers. For the membrane with two self-assembly bilayers of PDDA/PSS, 50% and 10% of vanadium ion permeability of a pristine SPFEK and Nafion 117 membranes can be afforded, respectively. Moreover, the oxidative stability of the PDDA/PSS-SPFEK composite membrane is improved remarkably compared with the pristine one. Consequently, the performance of VRBs using the PDDA/PSS-SPFEK composite membrane exhibits the highest coulombic efficiency (CE) of 82.1% at 30 mA cm^-2 and the longest duration stability in the self-discharge test.