pH‐Directed Phase Evolution in a Standalone Rust‐Derived Fe 2+ /Fe 3+ Redox Couple Catalyst for Efficient Oxygen Evolution in Alkaline and Seawater Media

ABSTRACT Converting naturally corroded rust into efficient electrocatalysts is fascinating but remains an underdeveloped approach for sustainable water splitting, primarily owing to inadequate phase regulation and restricted mechanistic insight. Here, we report a pH‐directed phase reconstruction strategy that converts waste rust into phase‐engineered iron oxides with tunable FeOOH, Fe 2 O 3 , and mixed Fe 2 O 3 /Fe 3 O 4 compositions. Controlled alkalinity (pH 14) leads to partial reduction of Fe 3+ to Fe 2+ during rust dissolution, enabling the formation of a mixed‐valence Fe 2+ /Fe 3+ state upon calcination. The optimized mixed‐phase catalyst (F3) delivers 306 ± 11 mV at 50 mA cm −2 for the OER, with a Tafel slope of 83 ± 6 mV dec −1 in alkaline water and 377 mV at 50 mA cm −2 with a Tafel slope of 90 mV dec −1 in alkaline seawater. EIS reveals significantly reduced R ct and enhanced admittance, correlating with mixed‐valence states and accelerated OER kinetics. A Faradaic efficiency of 96.7% confirms high oxygen selectivity in alkaline media. Multi‐batch reproducibility across three independent measurements confirms the robustness of the phase‐engineering pathway with 100 h long‐term stability. This work establishes a scalable waste‐to‐catalyst approach and reveals the mechanistic role of pH‐guided phase evolution in rust‐derived mixed‐valence systems as practical, low‐cost electrocatalysts for both alkaline and seawater electrolysis.