Scalable Alkaline Zinc-Iron/Nickel Hybrid Flow Battery with Energy Density up to 200 Wh L^-1

Achieving net-zero emissions requires low-cost and reliable energy storage devices that are essential to deploy renewables. Alkaline zinc-based flow batteries such as alkaline zinc-iron (or nickel) flow batteries are well suited for energy storage because of their high safety, high efficiency, and low cost. Nevertheless, their energy density is limited by the low solubility of ferro/ferricyanide and the limited areal capacity of sintered nickel electrodes. Here, combining the electrochemical reaction with the chemical reaction of ferro/ferricyanide couple in a homemade nickel electrode, an alkaline zinc-iron/nickel hybrid flow battery with a high energy density of 208.9 Wh L^-1 and an energy efficiency of 84.7% at a high current density of 80 mA cm^-2 is reported. The reversible chemical reactions between dual couples are proven to stabilize the nickel electrode by promoting the activation of the nickel electrode and further preventing the formation of γ-NiOOH. A kW-scale stack is demonstrated by the integration of ferro/ferricyanide couple with nickel electrode, delivering a coulombic efficiency of 98% and an energy efficiency of 89% at 40 mA cm^-2 . This work demonstrates a promising pathway for constructing and upscaling flow batteries with high energy density and low cost.