Conductive Membrane Coatings for High-Rate Vanadium Redox Flow Batteries

A conductive coating of carbon nanotubes (CNTs) and Nafion dispersion in water was deposited on a Nafion membrane via air-controlled electrospray. When the coated membrane was assembled into a large single cell of a vanadium redox flow battery (VRB) with a surface area of 35 cm², it was found that its cycling performance was greatly enhanced at much higher current densities than was afforded by the pristine Nafion membrane. A masking technique was also applied during the electrospraying process to create alternating domains of coated and uncoated membrane surfaces, which helped to mitigate the restriction of proton transport through the membrane due to the coating, while still decreasing the surface resistivity and thus the interfacial resistance of the membrane. Our results reveal that a very small mass of CNTs (∼0.015 mg CNT/cm²) enabled large improvements in the capacity retention and voltaic efficiencies of the vanadium redox battery during charging and discharging. This method has shown to be a reasonably fast, simple, and scalable technique for improving rate capability of VRBs, with the potential for extension to other redox flow battery systems.