Nitrogen, Phosphorus Co-Doped Graphite Felt as Highly Efficient Electrode for VO2+/VO2+ Reaction

All-vanadium redox flow batteries hold promise for the next-generation grid-level energy storage technology in the future. However, the low electrocatalytic activity of initial graphite felt constrains the development of VRFBs. Furthermore, the positive VO2+/VO2+ reaction involves complex multistep processes and more sluggish kinetics than negative V2+/V3+ reaction. Therefore, enhancing the kinetics of positive reaction is especially important. Heteroatom doping is one of the effective strategies for preparing carbon electrodes with high electrocatalytic activity and good stability. Here, a nitrogen, phosphorus co-doped graphite felt is prepared. Nitrogen introduces more negative charge into the carbon lattice due to the higher electronegativity, and more oxygen-containing functional groups will be introduced into the carbon lattice due to phosphorus-doped graphite felt. N, P co-doping provides more adsorption sites for vanadium ions. As a result, nitrogen, phosphorus co-doped graphite felt shows high electrochemical activity and good stability, and the corresponding VRFB presents a good voltage efficiency of 75% at a current density of 300 mA cm−2, which is 11% higher than the pristine graphite felt. During 100 charge/discharge cycles, the energy efficiency and voltage efficiency remain at 84% and 86% under the current density of 150 mA cm−2.