Geometry-Induced Asymmetric Vanadium-Ion Permeation of PVDF Membranes and Its Effect on the Performance of Vanadium Redox Flow Batteries
Seonghyun Yoon, Eun‐Ju Lee, Sang Jun Yoon, Duk Man Yu, Young Jin Kim, Young Taik Hong, Soonyong So
Abstract
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+).