A highly-efficient composite polybenzimidazole membrane for vanadium redox flow battery

The use of polybenzimidazole (PBI)-based membranes in vanadium redox flow battery (VRFB) exhibits a high coulombic efficiency due to their excellent ion selectivity, but a low energy efficiency resulting from the low proton conductivity. In this work, we propose and develop a composite membrane, comprising a dense but thin PBI layer, and a porous but thick layer made of PBI electrospun nanofibers. The dense layer, 7.0 μm thick, not only can retain the high ion selectivity, but also facilities the proton conduction as a result of the reduced thickness. The porous layer, 23 μm in thickness, 2 μm in pore size, is tightly stuck to the dense layer to ensure the mechanical stability. A battery with the composite membrane demonstrates an energy efficiency of 82.0% at 150 mA cm−2, nearly 20% higher than that with a conventional PBI membrane. More impressively, this membrane enables the battery to be stably operated for 200 cycles with a decent capacity retention of 99.81% per cycle, far surpassing the commercialized Nafion 211 membrane with a similar thickness. Both the remarkable battery performance and the preferable stability suggest that the composite PBI membrane is a promising alternative for VRFB applications.