4.7 Article

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

Journal

ACS APPLIED ENERGY MATERIALS
Volume 4, Issue 5, Pages 4473-4481

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/acsaem.1c00097

Keywords

PVDF; non-solvent-induced phase separation (NIPS); porous membrane; asymmetric permeability; vanadium redox flow battery

Funding

  1. Korea Research Institute of Chemical Technology Core Research Program [KS2022-20]
  2. Hydrogen Energy Innovation Technology Development Program of the National Research Foundation of Korea (NRF) - MSIT [NRF2019M3E6A1064729]

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In vanadium redox flow batteries, geometry-induced asymmetric ion permeation through porous membranes has been demonstrated, with higher vanadium-ion permeability observed when the finger-like top layer faces the vanadium-ion solution. This asymmetric ion selectivity based on membrane direction influenced the performance of VRFBs, showing better performance when the sponge-like bottom layer faces the anolyte side containing more permeable vanadium ions.
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 degrees 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+).

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