Layered composite membranes based on porous PVDF coated with a thin, dense PBI layer for vanadium redox flow batteries

A commercial porous polyvinylidene fluoride membrane (pore size 0.65 μm, nominally 125 μm thick) is spray coated with 1.2–4 μm thick layers of polybenzimidazole. The area resistance of the porous support is 36.4 mΩ cm² in 2 M sulfuric acid, in comparison to 540 mΩ cm² for a 27 μm thick acid doped polybenzimidazole membrane, and 124 mΩ cm² for PVDF-P20 (4 μm thick blocking layer). Addition of vanadium ions to the supporting electrolyte increases the resistance, but less than for Nafion. The expected reason is a change in the osmotic pressure when the ionic strength of the electrolyte is increased, reducing the water contents in the membrane. The orientation of the composite membranes has a strong impact. Lower permeability values are found when the blocking layer is oriented towards the vanadium-lean side in ex-situ measurements. Cells with the blocking layer on the positive side have significantly lower capacity fade, also much lower than cells using Nafion 212. The coulombic efficiency of cells with PVDF-PBI membranes (98.4%) is higher than that of cells using Nafion 212 (93.6%), whereas the voltage efficiency is just slightly lower, resulting in energy efficiencies of 85.1 and 83.3%, respectively, at 80 mA/cm².