Surface Modification of PAN-Derived Commercial Graphite Felts Using Deep Eutectic Solvents for their Application as Electrodes in All-Vanadium Redox Flow Batteries

All-vanadium redox flow batteries are promising large-scale energy storage solutions to support intermittent power generation. Commercial graphite felts are among the most used materials as electrodes for these batteries due to their cheap price, high conductivity, and large surface area. However, these materials exhibit poor wettability and electrochemical activity towards vanadium redox reactions, which translates into overpotentials and lower efficiencies. Deep eutectic solvents (DES) are mixtures of Lewis acids and bases that exhibit lower melting points than their original components. Here, a DES composed of choline chloride and urea, and a DES composed of FeCl3 and NH4Cl have been employed to modify the surface of graphite felts alongside a series of re-carbonization steps. The resulting materials were compared against pristine, thermally activated, and oxidatively activated graphite felts. Our results indicated that the treatments introduced new oxygen and nitrogen functionalities to the carbonaceous surface and increased the surface area, the degree of disorder and defects in the graphitic layers of the fibres. Cyclic voltammetry studies demonstrated higher electrochemical activity towards vanadium redox reactions and electrochemical impedance spectroscopy experiments showed the modified materials exhibited significantly lower charge transfer resistances. When tested in full cell configuration the electrode modified with the urea-based DES exhibited comparable coulombic efficiencies and superior energy storage capacity retention than the thermally oxidized felt used as benchmark, suggesting that the introduction of oxygen- and nitrogen-rich functional groups had a positive effect on the overall electrochemical performance of graphite felts.