A sub-10 μm Ion Conducting Membrane with an Ultralow Area Resistance for a High-Power Density Vanadium Flow Battery

With the outstanding features of high safety, high efficiency, and long lifespan, the vanadium flow battery (VFB) is well-suited for large-scale energy storage; however, it suffers from low power density. The high ion conductivity of membranes is very important to increase the performance of VFBs at high current densities and improve their power density. Here, we show a highly conductive free-standing sub-10 μm polybenzimidazole (PBI) membrane. The decrease in the membrane thickness contributes to shorter ion-transport pathways and lower resistance. The relatively loose cross-linked structure of the thin membrane provides sufficient free volume for ion transport. According to these results, the membrane exhibits an ultralow area resistance of 0.04 Ω cm2, much lower than that of commercial Nafion 115 membrane (0.20 Ω cm2), making the ion conductivity superior. Additionally, the sub-10 μm PBI membrane also shows a very high tensile strength of 45.5 MPa and high ion selectivity. The VFB assembled with a sub-10 μm PBI membrane delivers a high energy efficiency of approximately 80% at a high current density of 200 mA cm–2 and can run stably for more than 500 cycles without obvious performance decay. The increased performance of the VFB at a very high current density of 200 mA cm–2 contributes to its higher power density. Therefore, it is an available way to adopt free-standing sub-10 μm PBI membranes with high conductivity, selectivity, and mechanical stability to improve the power density of VFBs. Similarly, the application of it will also accelerate the practical application of VFB energy storage technology.