Graphene oxide nanoplatelets as excellent electrochemical active materials for VO²⁺/VO₂ ⁺ and V²⁺/V³⁺ redox couples for a vanadium redox flow battery

Graphene oxide nanoplatelets (GONPs) are presented as electrochemical active materials for VO²⁺/VO₂ ⁺ and V²⁺/V³⁺ redox couples for a vanadium redox flow battery. The structures and electrochemical properties of GONPs treated at different temperatures were investigated by transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy and cyclic voltammetry. The results indicate that GONPs treated at 50°C (GONP-50) possess highly hydroxylated and carboxylated groups and exhibit an excellent electrocatalytic activity towards VO²⁺/VO₂ ⁺ and V²⁺/V³⁺ redox couples, while the pristine graphite only shows a reversible electrocatalytic activity towards VO²⁺/VO₂ ⁺, suggesting that the V²⁺/V³⁺ redox reaction more strongly depends on the oxygen-containing groups attached on graphite surface than does the VO²⁺/VO₂ ⁺. With the increase of treatment temperature, the polarization is reduced significantly. GONPs treated at 120°C (GNOP-120) exhibit a lower electrochemical polarization than that of GONP-50 because of relatively higher electrical conductivity despite moderate electrocatalytic activity. The diffusion of VO²⁺ is faster on the surface of GONP-50 than on the pristine graphite and GNOP-120.