Graphite oxide-based graphene materials as positive electrodes in vanadium redox flow batteries

Two graphene materials, TRGO-1 and TRGO-2, prepared by the thermal exfoliation/reduction at 1000 °C of two graphite oxides with different characteristics, are investigated as positive electrodes in a vanadium redox flow battery (VRFB). A detailed study of their electrochemical response toward the [VO2+]/[VO2+] redox system is carried out through cyclic voltammetry, electrochemical impedance spectroscopy and charge/discharge experiments. As a consequence of the differences in the structure of the parent graphite oxides, TRGO-1 and TRGO-2 exhibit different structural and physicochemical properties resulting in significantly different electrochemical performances toward the vanadium redox reactions. TRGO-1 exhibits a markedly enhanced electrochemical activity (higher peak current densities and lower overpotentials) and a better kinetic reversibility toward the oxidation/reduction vanadium processes than TRGO-2. Furthermore, charge/discharge tests performed on two VRB single cells, the only differing component being the positive electrode, present higher coulombic, voltage and energy efficiency values in that battery containing the TRGO-1 electrode. The better results achieved with this sample are attributed to the higher degree of restoration of the 2D graphitic structure, and to the consequently higher electrical conductivity which increases the heterogeneous electron transfer rate. Moreover, residual hydroxyl groups present may act as active reaction sites and contribute to enhance its electrochemical response.