Controlled Construction of a N-Doped Carbon Nanotube Network Endows Carbon Felt with Superior Performances for High-Rate Vanadium Flow Batteries

Developing carbon felt (CF) electrodes with sufficient mass transfer channels and highly active catalytic interfaces remains a great challenge for high-rate vanadium flow batteries (VFBs). Herein, a well-defined 3D hierarchical N-doped carbon nanotube (NCNT) network is designed and grown onto CF via a facile bottom-up strategy, which features a high bonding strength, controllable growth morphology, and tunable electron structure. In the strategy, ZIF-67 arrays as both precursors and catalysts are self-assembled on CF followed by decomposition of melamine as an initiator into C and N sources for controlled growth of NCNTs during pyrolysis. By precisely regulating the microstructure of ZIF-67 precursors and the usage amount of melamine, the NCNT-modified CF composite electrode simultaneously achieves fast electron transport, facile mass transport, and high catalytic performance toward VO2+/VO2+ and V2+/V3+ redox reactions. Electrostatic potential calculations further indicate that N dopants alter the electronic structure of CNTs and serve as the preferential sites for the adsorption of vanadium ions to promote the redox kinetics. Consequently, the battery assembled with the composite electrodes exhibits an impressive energy efficiency of 76.6% at 300 mA cm–2 and demonstrates prolonged stability throughout 550 consecutive charge–discharge cycles at 200 mA cm–2. These encouraging achievements shed fresh insights into the controlled synthesis of CNTs onto CF for high-rate VFBs.