Stable, Efficient Iron Electrodeposition via Anion-Directed Control of Fe(II) Coordination

Iron production accounts for >90% of global metal production by weight, along with the highest energy consumption of any single manufacturing process. Aqueous electrodeposition technologies offer an attractive opportunity to economize and modularize iron production. However, a lack of molecular-level design rules, even for well-known additives, such as citrate, hinders the development of new electrolytes that can enable efficient metal production. We propose anion coordination strength as a descriptor for iron electrodeposition, with anions of intermediate coordination strength yielding a balanced trade-off between pH stability and Faradaic efficiency (FE). Guided by hard-soft-acid–base theory, we utilize citrate as a co-anion in Cl- and SO4-based electrolytes to demonstrate direct correlations between Fe2+ coordination, electrolyte stability, and the (electro)chemistry of iron deposits. Utilizing this new understanding, we develop optimized electrolytes that are stable at near-neutral pH (≥5) for up to 10 days and achieve >95% FE at 10 mA/cm2 with suppressed iron oxide formation. This stands in stark contrast to deposits generated in citrate-free electrolytes at lower pH. Overall, careful design of co-anions and the resulting Fe2+ complexes enable stable, efficient electrolytes for high-purity iron electrodeposition, representing a promising new direction for developing electrolytes for efficient iron production.