A gradient bi-functional graphene-based modified electrode for vanadium redox flow batteries

Vanadium redox flow batteries (VRFBs) are an ideal choice for large-scale energy storage because they have the advantages of long cycle life, flexible design and high safety. However, the poor electrocatalytic activity of carbon-based materials results in a large polarization resistance and energy loss during charge/discharge that greatly limits their commercial viability. Here we report a hybrid electrode with a gradient bi-functional oxygen-containing groups for VRFBs. It consists of a hybrid material of graphene oxide (GO), reduced graphene oxide (rGO) and graphene foam (GF) that combines materials with a high electrocatalytic activity (GO) and a high electrical conductivity (GF). One side is enriched with functional groups and provides preferential redox reversibility for VO2+/VO2+ and V3+/V2+ redox couples because of the electrocatalytic nature of the many oxygen functional groups. The side with the low concentration of functional groups has a high electrical conductivity and facilitates electron transfer. As a result, this VRFB electrode achieves a low polarization, high discharge capacity, high energy density, and high energy efficiency and has great promise for use in VRFBs.