Geometry-Induced Asymmetric Vanadium-Ion Permeation of PVDF Membranes and Its Effect on the Performance of Vanadium Redox Flow Batteries

In vanadium redox flow batteries (VRFBs), size-exclusive porous separators are of great interest as alternative membranes to the conventional ion-exchange membranes. In this study, we have shown geometry-induced asymmetric vanadium permeation through porous poly(vinylidene fluoride) (PVDF) membranes having asymmetric pore structures. The asymmetric pore geometry was developed via non-solvent-induced phase separation (NIPS) of the PVDF/N,N-dimethylacetamide film in water at various coagulation bath temperatures (30–70 °C). The membranes show two distinct regions across their thickness direction, finger-like and sponge-like structures near the top and bottom of the membranes. In the permeability experiments, vanadium-ion (VO2+) permeability through a fixed asymmetric membrane was significantly affected by the direction of the VO2+ concentration gradient. The vanadium-ion permeation was higher when the finger-like top layer of the membrane faced the vanadium-ion solution than the other direction, while proton permeation was almost identical regardless of the direction. In single-cell tests, this geometry-induced asymmetric ion selectivity resulted in different performances depending on the direction of the membranes; VRFBs performed better when the sponge-like bottom layer faces the anolyte side, which contains more permeable vanadium ions (VO2+, VO2+) through porous membranes than other ions (V2+, V3+).