Deciphering the exceptional kinetics of hierarchical nitrogen-doped carbon electrodes for highperformance vanadium redox flow batteries

High-performance vanadium redox flow batteries (VRFBs) necessitate robust carbon electrodes, whose rational design demands quantitative relationships between the electrode properties and performance. Here, we decipher the exceptional kinetics of VO2⁺/VO²⁺on a hierarchical nitrogen-doped carbon (HNC) electrode. Diffusion-less cyclic voltammetry, a method developed to evade the complex influence of diffusion in three-dimensional porosity, quantifies a rate constant of ∼5 × 10^-7cm s^-1on the electrode, substantially higher than that of un-doped porous carbons, in line with the calculated adsorption energies of solvated vanadium cations. The hierarchy is further linked to the high specific area via a comparison with graphite felt with and without carbon nanotube-decoration. The HNC electrode enables a VRFB of an exceptional energy efficiency of 76.8% over 2000 cycles at 400 mA cm^-2, among the best reported. The work offers deep insights into the relationship of heteroatom doping, structural hierarchy, and kinetics for porous carbon electrodes for developing next-generation flow batteries.