Vanadium redox flow batteries using new mesoporous nitrogen-doped carbon coated graphite felt electrode

• Mesoporous N-doped carbons formed using cheap SC and urea are loaded onto GF. • SC and urea crosslinked within SC/U-GF induce its high loading onto GF. • SC/U-GF promotes reactivity for VIRR and mass transfer of vanadium ions. • VRFB using SC/U-GF shows excellent performance of EE of 71.6% at 200 mAcm -2. • VRFB using SC/U-GF preserves 99.6% of initial EE after 300 cycle at 200 mAcm -2. New and economical mesoporous nitrogen (N)-doped carbon structure is prepared using sodium citrate (SC) and urea precursors and then doped onto graphite felt (GF) electrode (SC/U-GF). The catalytic properties of SC/U-GF electrode are compared with those of pristine GF and SC-GF electrodes. When a large amount of mesoporous N-doped carbon is doped onto GF, the catalytic activity of SC/U-GF for improving the reactivity and reversibility of VO 2+ /VO 2 + and V 2+ /V 3+ redox reactions is excellent, and its proper mesoporous structure promotes the mass transfer of vanadium ions. This is proved by measuring its low charge transfer resistance and high active area. When VRFB single cell using SC/U-GF is tested, its energy efficiency (EE) is 80.2 % at 120 mA cm −2 , which is 7.8 and 3.1% better than VRFBs using pristine GF (72.4%) and SC-GF (77.1%). Even in a high current density (200 mA cm -2 ), its EE (71.6 %) is better than VRFBs using pristine GF (59.9 %) and SC-GF (66.4 %). Additionally, EE of VRFBs using SC-GF and SC/U-GF is well maintained (95.6 and 99.6 % preservation after 300 cycles). Based on that, it is obvious that SC/U-GF electrode strongly affects the enhancements in performance and stability of VRFB.