Revealing the stabilization pathway, stack validation, and techno-economic assessment of phosphonate-Fe(III) complex for near-neutral aqueous all-iron flow batteries

Abstract Aqueous iron-based redox flow batteries are promising for grid-scale energy storage due to inherent safety, low cost, and material abundance. However, their stability remains limited by poor redox reversibility and iron dendrite formation. Herein, we report a hexadentate phosphate-rich Fe(III) complex, Fe(P4N2), specifically engineered for near-neutral conditions. The robust Fe–O/Fe–N coordination environment of phosphate-based ligand framework stabilizes redox-active Fe3+/Fe2+ centers and suppresses parasitic side reactions. Additionally, the hydrogen-bond-rich solvation shell, formed by abundant phosphonate terminal groups, fosters a dense hydration layer around the complex, bringing high aqueous solubility (1.47 M) and low cross-membrane contamination. Consequently, the Fe(P4N2)-based flow batteries demonstrate high energy efficiency (85.7% at 100 mA cm−2), rapid redox kinetics (21.4 Wh L−1 at 200 mA cm−2), ultralow capacity decay (0.00052% per cycle or 0.027% per day). When scaled to a 12-cell stack (900 cm2 active area per cell), the Fe(P4N2)-based flow batteries deliver a stable 300 W power output during long-term operation. Techno-economic analysis demonstrates promising economic potential for this system, driven by low material cost, high energy efficiency, and long-term durability. This study establishes a multidentate phosphate coordination strategy for developing fast-charging, durable, scalable, and economically attractive all-iron redox flow batteries.