Accelerating the Reduction Kinetics of V⁴⁺ to V³⁺ on Atomically Fe─N₄ Decorated Carbon Nanotubes for Vanadium Electrolyte Preparation

The high manufacturing cost of vanadium electrolytes is caused by the sluggish kinetics of V⁴⁺ to V³⁺, which restricts the commercialization of all vanadium flow batteries (VFBs). Here, density functional theory calculations first reveal the detailed reaction pathway and point out the rate-determined step by the desorption of the end product [V(H₂O)₆]³⁺. Catalytic site engineering at the molecular level can optimize the adsorption energy of [V(H₂O)₆]³⁺ to boost the kinetics. Furthermore, iron single-atoms embedded nitrogen-doped carbon nanotubes (FeSA/NCNT) are designed to decrease the adsorption energy of [V(H₂O)₆]³⁺. The reaction rate constant of FeSA/NCNT toward V⁴⁺ to V³⁺ is 1.62 × 10^-7 cm s^-1, 37.5 times that of the commercial carbon catalyst. Therefore, the energy consumption is reduced by 22.5%. Meanwhile, the prepared vanadium electrolyte is of high quality with the ideal oxidation state of + 3.5 without impurities. This work reveals the catalytic mechanism of V⁴⁺ to V³⁺ and proposes a simple but practical strategy to reduce the preparation cost of V^3.5+ electrolyte.