Engineering Ligand-Functionalized Layers for Enhanced Fe²⁺ Capture and Stable Deposition in All-Iron Flow Batteries

All-iron aqueous redox flow battery (ARFB) is a promising candidate for next-generation large-scale energy storage, due to low cost and environmental benignity, but the low reversibility of the Fe anode limits its long-term cycling stability. Herein, a 3-mercaptopropionic acid (MPA) ligand-functionalized layer (LFL) is utilized to achieve a highly reversible Fe-MPA anode. The MPA LFL not only minimizes corrosion and hydrogen evolution in acidic electrolyte, but also actively captures Fe2+ ions and controls interfacial deposition. Specifically, the MPA LFL homogenizes Fe2+ ion flux at the electrode interface, acting as a regulator that suppresses the dendritic iron growth and facilitates the reversible anode plating/stripping. Consequently, the all-iron ARFB based on the Fe-MPA anode delivers high Coulombic efficiency of ∼99.5% and energy efficiency of ∼75% over 330 h at 20 mA cm–2. This study highlights the strategy of designing function-oriented ligand-electrode interface for improving the stability of ARFBs.