Superior electrocatalytic negative electrode with tailored nitrogen functional group for vanadium redox flow battery

Development of electrodes with high electrocatalytic activity and stability is essential for solving problems that still restrict the extensive application of vanadium redox flow batteries (VRFBs). Here, we designed a novel negative electrode with superior electrocatalytic activity by tailoring nitrogen functional groups, such as newly formed nitro and pyridinic-N transformed to pyridonic-N, from the pre-nitrogen-doped electrode. It was experimentally confirmed that an electrode with pyridonic-N and nitro fuctional groups (tailored nitrogen-doped graphite felt, TNGF) has superior electrocatalytic acivity with enhanced electron and mass transfer. Density functional theory calulations demonstrated the pyridonic-N and nitro functional groups promoted the adsorption, charge transfer, and bond formation with the vanadium species, which is consistent with expermental results. In addition, the V²⁺/V³⁺ redox reaction mechanism on pyridonic-N and nitro functional groups was estabilised based on density functional theory (DFT) results. When TNGF was applied to a VRFB, it enabled enhanced-electrolyte utilization and energy efficiencies (EE) of 57.9% and 64.6%, respectively, at a current density of 250 mA cm⁻². These results are 18.6% and 8.9% higher than those of VRFB with electrode containing graphitic-N and pyridinic-N groups. Interestingly, TNGF-based VRFB still operated with an EE of 59% at a high current density of 300 mA cm⁻². The TNGF-based VRFB exhibited stable cycling performance without noticeable decay of EE over 450 charge–discharge cycles at a current density of 250 mA cm⁻². The results of this study suggest that introducing pyridonic-N and nitro groups on the electrode is effective for improving the electrochemical performance of VRFBs.