Sulfonated Polystyrene/Polybenzimidazole Bilayer Membranes for Vanadium Redox Flow Batteries

Abstract

Polybenzimidazole has been widely examined as a separator for vanadium redox flow batteries (VRFBs) due to its low vanadium permeability. Its low conductivity can be tackled by combining 1-4 mu m thin dense PBI layers with highly conductive mechanically supporting layers, either by lamination or loose stacking. While gel-PBI is very soft and conductive, the shadow effect of the non-conductive pore walls of porous supports adds resistance. In this work, these issues are addressed by coating a 25 mu m thick highly conductive sulfonated polystyrene layer (S) with a 1 mu m thin selective PBI layer (P) to block vanadium crossover. To reduce the number of potential defects, two bilayer membranes can be stacked. A 52 mu m thick stack of two membranes (PS-SP, PBI faces the electrodes) shows an area-specific resistance of 144.8 m ohm 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. A VRFB cell test over 3500 charging cycles (1660 h) with an energy efficiency of up to 88.5% at 100 mA cm-2 is shown. 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 at low costs. PBI-coated sulfonated polystyrene membranes are prepared, and the performance of the various possible assemblies of two such membranes is tested. A 52 mu m thick PSSP assembly can be fabricated with a materials cost of just 1.84 USD m-2, and shows an energy efficiency of 89.7% at 80 mA cm-2. image