Selective anion exchange membranes for high coulombic efficiency vanadium redox flow batteries

A quaternary ammonium functionalized poly(fluorenyl ether) anion exchange membrane (AEM) with extremely low VO^{2 +} permeation was characterized for vanadium redox flow battery (VRFB) application. One hundred percent coulombic efficiency (CE) was achieved for the AEM-based VRFB at all the current densities tested. Comparatively, the CE of a N212 membrane-based VRFB was lower than 94% and varied with charge/discharge current density. At current densities lower than 60 mA cm^{- 2}, the energy effiency of the AEM-based VRFB was higher than that of a device with N212. The cycling performance demonstrated that the AEM-based VRFB was free of capacity fade, which is a consequence of its low VO^{2 +} permeability. These observations are of significant importance for flow batteries that operate intermittently or at moderate current densities.