Chloride Ion‐Specific Etching‐Driven Synthesis of Porous Cobalt‐Doped FeOOH Anodes for Robust Ni‐Fe Batteries

Rechargeable nickel-iron (Ni-Fe) batteries are gaining renewed attention for next-generation energy storage due to their inherent safety, low cost, and stable discharge profile. However, their performance is often hindered by limitations of the iron anode, such as low surface area, poor structural integrity, and sluggish redox kinetics. This work developed a chloride-ion-assisted etching strategy to construct cobalt-doped iron oxyhydroxide (Co-FeOOH) porous nanosheets on Fe foam in situ. Chloride ions, with their strong corrosive capability and small hydrated radius, rapidly broke down the passive oxide layer, forming a highly porous structure rich in active sites. The resulting electrode delivered an areal capacity of 1.4 mAh cm^-2 at 4 mA cm^-2, outperforming analogues treated with nitrate and sulfate ions. Assembled with a sulfur-doped Ni-Mn hydroxide cathode, the full cell achieved 0.9 mAh cm^-2 at 10 mA cm^-2, surpassing many current aqueous battery systems. This study presents a targeted etching approach for optimizing iron anodes, offering a viable path toward safer, low-cost, and long-life Ni-Fe batteries.