High Performance and Long Cycle Life Neutral Zinc-Iron Flow Batteries Enabled by Zinc-Bromide Complexation

Zinc-based flow batteries have attracted tremendous attention owing to their outstanding advantages of high theoretical gravimetric capacity, low electrochemical potential, rich abundance, and low cost of metallic zinc. Among which, zinc-iron (Zn/Fe) flow batteries show great promise for grid-scale energy storage. However, they still face challenges associated with the corrosive and environmental pollution of acid and alkaline electrolytes, hydrolysis reactions of iron species, poor reversibility and stability of Zn/Zn2+ redox couple. In this work, bromide ions are used to stabilize zinc ions via complexation interactions in the cost-effective and eco-friendly neutral electrolyte. Cyclic voltammetry results reveal that the redox reversibility between Zn and stabilized Zn2+ is greatly improved. The results of spectrum characterizations and density functional theory calculations verify that the formation of Zn[Br-n(H2O)(6-n)](2-n) (1 <= n <= 4, n is integer.) ions accounts for the increased electrochemical reversibility of Zn/Zn2+ pair. Moreover, to overcome the bottleneck of slow kinetics of the coordination interactions between Zn2+ and Br-, ZnBr2 is judiciously selected as the electrolyte additive to promote the complexation process. Adopting K3Fe(CN)(6) as the positive redox species to pair with the zinc anode with ZnBr2 modified electrolyte, the proposed neutral Zn/Fe flow batteries deliver excellent efficiencies and superior cycling stability over 2000 cycles (356 h), shedding light on their great potential for large scale energy storage.