A Neutral Zinc–Iron Flow Battery with Long Lifespan and High Power Density

Neutral zinc–iron flow batteries (ZIFBs) remain attractive due to features of low cost, abundant reserves, and mild operating medium. However, the ZIFBs based on Fe(CN)63–/Fe(CN)64– catholyte suffer from Zn2Fe(CN)6 precipitation due to the Zn2+ crossover from the anolyte. Even worse, the opposite charge properties of positive and negative active species enable a big contradiction in the design of a suitable membrane. Herein, sodium citrate (Cit) was introduced to coordinate with Zn2+, which effectively alleviated the crossover and precipitation issues. Meanwhile, the redox species exhibited considerable kinetics and reversibility with a good capability of hydrogen evolution inhibition. As a result, the assembled battery demonstrated a high energy efficiency of 89.5% at 40 mA cm–2 and operated for 400 cycles with an average Coulombic efficiency of 99.8%. Even at 100 mA cm–2, the battery showed an energy efficiency of over 80%. This paper provides a possible solution toward a low-cost and sustainable grid energy storage.