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 V3.5+ electrolyte.

A Double-ligand Chelating Strategy to Iron Complex Anolytes with Ultrahigh Cyclability for Aqueous Iron Flow Batteries.

Aqueous all-iron flow batteries (AIFBs) are attractive for large-scale and long-term energy storage due to their extremely low cost and safety features. To accelerate commercial application, a long cyclable and reversible iron anolyte is expected to address the critical barriers, namely iron dendrite growth and hydrogen evolution reaction (HER). Herein, we report a robust iron complex with triethanolamine (TEA) and 2-methylimidazole (MM) double ligands. By introducing two ligands into one iron center, the binding energy of the complex increases, making it more stable in the charge-discharge reactions. The Fe(TEA)MM complex achieves reversible and stable redox between Fe3+ and Fe2+ , without metallic iron growth and HER. AIFBs based on this anolyte perform a high energy efficiency of 80.5 % at 80 mA cm-2 and exhibit a record durability among reported AIFBs. The efficiency and capacity retain nearly 100 % after 1,400 cycles. The capital cost of this AIFB is $ 33.2 kWh-1 (e.g., 20 h duration), cheaper than Li-ion battery and vanadium flow battery. This double-ligand chelating strategy not only solves the current problems faced by AIFBs, but also provides an insight for further improving the cycling stability of other flow batteries.