Thermo–chemical treatments based on NH3/O2 for improved graphite-based fiber electrodes in vanadium redox flow batteries

Electrochemical behavior of the polyacrylonitrile (PAN)-based graphite as a low cost electrode material for vanadium based redox batteries (VFB) in sulfuric acid medium has been improved by means of the successful introduction of nitrogen and oxygen-containing groups at the graphite surface by thermal activation under NH3/O2 (1:1) atmosphere. Influence of the temperature and treatment duration times have been studied towards the positive reaction of VFB. The structure, composition, and electrochemical properties of the treated samples have been characterized with field emission scanning electron microscopy, X-ray photoelectron spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy. The estimation of electrochemical surface area has also been evaluated. The treatment of PAN graphite material at 773 K for 24-h leads to electrode materials with the best electrochemical activity towards the VO2+/VO2+ redox couple. This method produces an increase of the nitrogen and oxygen content at the surface up to 8% and 32%, respectively, and is proved to be a straightforward and cost-effective methodology. This improvement of the electrochemical properties is attributed to the incorporation of the nitrogen and oxygen-containing groups that facilitate the electron transfer through the electrode/electrolyte interface for both oxidation and reduction processes.