Highly efficient vanadium redox flow batteries enabled by a trilayer polybenzimidazole membrane assembly

Abstract A novel polybenzimidazole (PBI)‐based trilayer membrane assembly is developed for application in vanadium redox flow battery (VRFB). The membrane comprises a 1 µm thin cross‐linked poly[2,2′‐( p ‐oxydiphenylene)−5,5′‐bibenzimidazole] (OPBI) sandwiched between two 20 µm thick porous OPBI membranes (p‐OPBI) without further lamination steps. The trilayer membrane demonstrates exceptional properties, such as high conductivity and low area‐specific resistance (ASR) of 51 mS cm −1 and 81 mΩ cm 2 , respectively. Contact with vanadium electrolyte increases the ASR of trilayer membrane only to 158 mΩ cm 2 , while that of Nafion is 193 mΩ cm 2 . VO 2+ permeability is 2.73 × 10 −9 cm 2 min −1 , about 150 times lower than that of Nafion NR212. In addition, the membrane has high mechanical strength and high chemical stability against VO 2 + . In VRFB, the combination of low resistance and low vanadium permeability results in excellent performance, revealing high Coulombic efficiency (>99%), high energy efficiency (EE; 90.8% at current density of 80 mA cm −2 ), and long‐term durability. The EE is one of the best reported to date.