Sulfonated Polystyrene/Polybenzimidazole Bilayer Membranes for Vanadium Redox Flow Batteries

Abstract

Polybenzimidazole has been intensively investigated as membrane material for vanadium redox flow batteries (VRFBs) due to its low vanadium permeability. While PBI exhibits low conductivity1, the membrane resistance can be minimized by combining 1-4 ？m thin PBI layers with highly conductive supporting membranes, either by direct lamination or loose stacking. Gel-PBI was used as support but is very soft2, and the shadow effect of the non-conductive pore walls of porous membranes such as porous PVDF3 adds to the total resistance. In this work, these issues are resolved by making a monolithic bilayer membrane, consisting of a 25 ？m thick highly conductive sulfonated polystyrene layer (S) with a 1 ？m thin selective PBI layer (P) hindering the vanadium crossover4. To reduce the number of potential defects, two bilayer membranes can be stacked. A 52 μm thick stack of two membranes (PS-SP, PBI faces the electrodes) shows an area specific resistance of 144.8 mΩ cm2 in VO2+-containing electrolyte and a permeability of 6.85 x 10-14 m2 s-1, both lower than the values for Nafion 212. The membrane showed an excellent durability in a VRFB cell test for over 3500 charging cycles (>1660 h) with an energy efficiency up to 88.5% at 100 mA cm-2. Capacity and performance losses are reversed by electrolyte rebalancing. With material costs of 1.84 USD m-2, the PSSP (1-25-25-1) membrane promises high performance and durable operation for VRFBs at a very low costs of material investment.