Ligand engineering enables an alkaline iron-chromium redox flow battery

Iron-chromium redox flow batteries are promising for long-duration energy storage owing to their safety, low cost, and environmental compatibility, but are limited by sluggish chromium(II/III) kinetics and parasitic hydrogen evolution in chromium-based negolytes. Here we re-engineer iron-chromium redox chemistry through chelation-driven molecular design using the strong-field ligand 2,2-Bis(hydroxymethyl)-2,2',2"-nitrilotriethanol. The ligand reshapes the electronic configurations of iron and chromium centres and tunes their redox potentials, enabling an alkaline iron-chromium redox flow battery with rapid kinetics and suppressed hydrogen evolution. It stabilizes an octahedral hexacoordinate chromium chelate and unlocks an unconventional chromium(III/IV) couple at +0.34 V versus the standard hydrogen electrode, circumventing the Jahn-Teller effects associated with chromium(II/III). Paired with an iron-chelate negolyte based on the same ligand, the cell operates at 1.15 V within the aqueous stability window and remains stable over 500 cycles, achieving 99.0% coulombic efficiency and 75.8% energy efficiency at 100 mA cm^-2 (80% of state-of-charge). Together with a projected raw-material cost of $17.02 kWh^-1 and theoretical capacity of 68.07 Ah L^-1 based on posolyte volume, this work establishes a scalable, cost-effective strategy for grid-scale energy storage.