Composite Membranes Containing a Porous Separator and a Polybenzimidazole Thin Film for Vanadium Redox Flow Batteries

Abstract

Redox flow batteries (RFBs) are energy storage devices designed for grid-scale application. For next generation RFBs it is desirable to develop low cost materials with low ohmic resistance and high transport selectivity. We present a composite membrane for the vanadium redox flow battery (VRFB) consisting of a composite of a porous polypropylene separator laminated with a thin film of polybenzimidazole (PBI). PBI layers are prepared by solution casting to obtain thicknesses in the range of 0.2 to 10 μ m. The ohmic resistance of vanadium electrolyte imbibed PBI is ∼50 mOhm·cm 2 per micrometer of film thickness at room temperature. In cell tests, composite membranes show higher coulombic efficiency compared to Nafion® 212. Composite membranes with a PBI layer thickness of 1 μ m and below outperform Nafion® 212 in terms of energy efficiency and discharge capacity up to a current density of 250 mA cm −2 . With thicker PBI films the ohmic cell resistance is excessively high. Over 100 charge-discharge cycles a higher rate of capacity fading is observed for a composite membrane with 0.7 μ m PBI compared to Nafion® 212, which is a result of a more pronounced net electrolyte flux from the negative to the positive electrolyte.