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

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 cm², respectively. Contact with vanadium electrolyte increases the ASR of trilayer membrane only to 158 mΩ cm², while that of Nafion is 193 mΩ cm². VO²⁺ permeability is 2.73 × 10⁻⁹ cm² min⁻¹, about 150 times lower than that of Nafion NR212. In addition, the membrane has high mechanical strength and high chemical stability against VO₂⁺. 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.