Trilayer polybenzimidazole membrane assembly for highly efficient vanadium redox flow batteries

Abstract

Vanadium redox flow battery is one of the best options to store intermittent renewable energy because it demonstrates intrinsic safety, long lifespan, scalability, and high recyclability.[1] Although Nafion membrane is commonly used in VRFBs to separate anolyte and catholyte, it is expensive and exhibits high vanadium crossover.[2] Herein, we present an alternative novel polybenzimidazole based trilayer membrane assembly for use in VRFBs. The assembly consists of a highly selective 1 ？m thin cross-linked poly[2,2’-(p-oxydiphenylene)-5,5’- bibenzimidazole] (OPBI) membrane sandwiched between two highly conductive 20 ？m thick porous OPBI membranes (p-OPBI) as protective layers. Notably, this trilayer membrane assembly exhibits remarkable properties including high conductivity and a low area specific resistance (ASR) of 51 mS cm-1 and 81 mΩ cm2 , respectively. In contact with vanadium electrolyte, the ASR of the membrane assembly only increases to 158 mΩ cm2 , whereas the ASR of Nafion reaches 193 mΩ cm2 . VO2+ permeability is 2.73x10-9 cm2 min-1 , about 150 times lower than that of Nafion NR212. In addition, the membrane exhibits remarkable mechanical robustness and high chemical stability against VO2 +. In the VRFB, the low resistance and low vanadium permeability translates into excellent performance, revealing high coulombic efficiency (CE) (> 99%), high energy efficiency (EE) at the world level (90.8% at current density of 80 mA cm-2 ), and long-term durability.[3]