Unraveling the relevance of carbon felts surface modification during electrophoretic deposition of nanocarbons on their performance as electrodes for the VO2+/VO2+redox couple

The energy efficiency of vanadium redox flow batteries (VRFB) greatly depends on the electrochemical performance of the electrodes that constitute these devices, so it is essential to develop more efficient electrode materials with optimized structure and physicochemical properties. Carbon-based felts are commonly used as electrodes in such devices due to their high chemical stability in acid media, high electrical conductivity and their three-dimensional structure. However, they show low reversibility and electrocatalytic activity, especially for the VO2+ / VO2+ redox couple. In the quest for improving the electrochemical behaviour of carbon-based felts, it has been proposed to modify them with functionalized carbon nanotubes or graphene oxide by electrophoretic deposition. To elucidate if the improvements achieved on these electrodes can be really ascribed to the presence of the carbon nanomaterials or to the oxidation undergone by the fibers during this treatment, the electro-oxidation of a pristine rayon-based graphitized carbon felt was performed in parallel. The electro-oxidized felt exhibits the best behaviour, suggesting that the improved performance obtained for the electrodes modified with nanocarbons are mainly influenced by the oxidation of the fibers.