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

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 μm. The ohmic resistance of vanadium electrolyte imbibed PBI is ∼50 mOhm·cm² 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 μm 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 μm PBI compared to Nafion® 212, which is a result of a more pronounced net electrolyte flux from the negative to the positive electrolyte.