In Situ Preparation of Ultra‐Thin and High‐Entropy FeCoNiMnMo Nanosheet Arrays to Achieve Efficient Water/Seawater Oxidation at Large Current Density

High entropy hydroxide is a promising catalyst for the oxygen evolution reaction (OER) due to its high entropy effect as well as unique structure. Whereas high preparation costs and the problem of competing chlorine evolution reaction in seawater electrolysis pose challenges for industrial applications. Herein, high-entropy FeCoNiMnMo-OH nanosheets are prepared on nickel foam substrate via a facile one-step room-temperature corrosion engineering strategy. The unique morphology can effectively increase the active sites and enrich OH^- in the cavity composed of the nanosheets, which significantly increases the local alkalinity and accelerates the actual reaction rate, combined with the inherent synergistic effect of multiple elements and structural stability of high-entropy system. These combined advantages enable exceptional OER performance in both alkaline electrolyte (247 mV at 100 mA cm^-2) and alkaline seawater (287 mV at 100 mA cm^-2). Furthermore, it also shows excellent long-term stability (stable OER activity at 100/500 mA cm^-2 for 100 h) across different media, demonstrating remarkable durability and chloride corrosion. This study establishes a material design paradigm for developing cost-effective, high-performance multi-metallic hydroxides through corrosion engineering, providing new insights into overcoming the activity-stability-cost trilemma in seawater electrolysis systems.