Graphene oxide nanosheets/polymer binders as superior electrocatalytic materials for vanadium bromide redox flow batteries

Few layered graphene oxide (GO) nanosheets with large specific surface area (42.1 m² g^-1) are successfully prepared by a modified Hummers method for use as electrodes in the vanadium bromide redox battery. The structure and physicochemical properties of GO are investigated by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. Cyclic voltammetry results indicate that GO nanosheets with polymer binder (i.e., polyvinylidiene fluoride (PVDF) or sulfonated poly(ether ether ketone) (SPEEK)) hybrids demonstrate more favorable electrocatalytic activity towards the Br^-/Br^3- and V³⁺/V²⁺ redox couples than the pure graphite. This is attributed to the large numbers of oxygen-containing functional groups on the GO nanosheet surface which can generate more active sites to catalyze the redox reactions. For the binder-based electrodes, the SPEEK based electrode gives the best electrochemical performance (e.g., lower overvoltage for both Br^-/Br^3- and V³⁺/V²⁺ redox couple reactions and higher peak currents for the V³⁺/V²⁺ redox couple).