Porous polybenzimidazole membranes with positive charges enable an excellent anti-fouling ability for vanadium-methylene blue flow battery

A cost-effective, high-performance and highly stable membrane has always been in intensively needed in aqueous organic-based flow batteries. Here we present a porous polybenzimidazole (PBI) membrane with positive charges that endow the membrane with a high rejection and an excellent anti-fouling ability for target organic molecule and asymmetric structure that affords a high conductivity for vanadium-methylene blue flow battery (V-MB FB). The morphologies and thickness of separating layer in particular of the porous PBI can be well adjusted by simply altering the polymer concentration in the cast solution and further afford the membrane with a controllable property in terms of both ion selectivity and ion conductivity. As a result, a V-MB FB assembled with a porous PBI membrane delivers a coulombic efficiency (CE) of 99.45% and an energy efficiency (EE) of 86.10% at a current density of 40 mA cm−2, which is 12% higher than that afforded by a Nafion 212 membrane. Most importantly, the V-MB FB demonstrates a methylene blue (MB) utilization of 97.55% at a theoretical capacity of 32.16 Ah L−1 (based on the concentration of MB in the electrolyte) because of the high ion conductivity of the membrane, which favors reducing the cost of a battery. The results suggest that the designed porous PBI membranes exhibit a very promising prospect for methylene blue-vanadium flow battery.