Exfoliated Graphene Composite Membrane for the All-Vanadium Redox Flow Battery

Vanadium redox flow batteries are emerging as a promising grid storage solution. Unlike competing flow battery concepts, these utilize vanadium in both the catholyte and anolyte chambers which enables easy regeneration and balancing of the cell upon crossover of species through the membrane during long-term use. To increase the time between regeneration cycles and to improve the overall efficiency of vanadium flow batteries, we investigate the use of an ultrathin, graphene coating on the surface of various Nafion membranes. Electrochemically exfoliated graphene (EEG) was dispersed at the air–water interface of a Langmuir–Blodgett trough, compressed, and transferred to Nafion 117 (180 μm thickness) and Nafion 115 (127 μm) membranes. Single-cell vanadium redox flow batteries assembled with the coated membranes led to significantly higher energy efficiency (increased by 13%), power density (by 67%), and discharge capacity (by 17.5%) over 100 cycles compared to uncoated Nafion. The graphene layer was stable over cycling, and electrochemical impedance spectroscopy and self-discharge experiments indicated that the improved battery performance is due to a combination of reduced vanadium crossover and enhanced electrochemical activity provided by the graphene at the electrode surface.