Crystal‐Structure‐Dependent Water Adsorption Governs Water Oxidation Activity of FeOOH Polymorphs: Insights from Electrochemical and Bias‐Free Photochemical Systems

FeOOH has emerged as a promising earth‐abundant catalyst for water oxidation, with activity comparable to that of precious‐metal oxides such as IrO 2 and RuO 2 under optimal conditions. Herein, we systematically investigate the effect of crystal structure on water oxidation activity using four FeOOH polymorphs (α, β, γ, and δ) under identical conditions. We demonstrate that the catalytic performance strongly depends on the crystal structure, with higher surface hydrophilicity leading to enhanced water oxidation activity. The same structure‐dependent trend is observed under both electrochemical and photochemical conditions, indicating that the intrinsic catalytic properties of FeOOH govern the reaction irrespective of the reaction mode. Density functional theory calculations reveal that the adsorption energy for H 2 O on FeOOH surfaces correlates well with the experimentally observed activity, with δ‐FeOOH exhibiting the highest performance. These findings identify interfacial hydrophilicity as a key descriptor for water oxidation on FeOOH polymorphs and provide design guidelines for the development of efficient, earth‐abundant water oxidation catalysts.