Regulating the N/B ratio to construct B, N co-doped carbon nanotubes on carbon felt for high-performance vanadium redox flow batteries

The heteroatom-doped carbon nanotubes hold great promise for improving the properties of carbon felt in vanadium redox flow batteries. However, the structure control and catalytic mechanisms of B, N co-doping are still insufficient. Herein, the precisely controllable B, N co-doped carbon nanotubes (BNCNTs) are constructed on the surface of carbon felt via an in-situ growth method. DFT calculations emphasize the significance of B, N codoping based on electron transfer promotion and vanadium adsorption enhancement. Subsequently, we regulate the N/B ratio (atomic ratio of N to B) to construct the BNCNTs with a reasonable tubular structure, and the optimal N/B ratio considerably enhances the electrode reaction kinetics. Benefiting from BNCNTs with the optimal N/B ratio, the battery adopting the prepared electrode can deliver an energy efficiency of 79.3% at 300 mA cm-2, and even achieve 68.7% energy efficiency at 500 mA cm-2. Furthermore, the batteries maintain an energy efficiency of around 80% at 300 mA cm-2 during 1000 long-term cycles, which demonstrates remarkable long-term cycle performance. This study reveals the catalytic mechanism of BNCNTs for V2+/V3+ and VO2+/VO2 + redox reactions, which can serve as a fundamental guide for further precise design of heteroatom co-doped carbon nanotubes electrode.