Cu‐Doped Amorphous High‐Entropy Hydroxides Activating Electrochemical Reconstruction: Triggering <scp>LOM</scp> and Chloride Repellency for Enhanced Water and Seawater Oxidation

The development of oxygen evolution reaction (OER) electrocatalysts with exceptional activity and robust corrosion resistance is pivotal for advancing water and seawater electrolysis toward sustainable hydrogen production. Herein, copper‐doped amorphous high‐entropy hydroxide (CuNiCoFeCrOH) is constructed via one‐step electrodeposition on nickel foam, specifically designed to enhance water/seawater oxidation performance. Comprehensive electrochemical analyses further demonstrate that the Cu doping significantly promotes electrochemical reconstruction by accelerating Cr leaching and re‐adsorption, which boosts active oxyhydroxide formation, optimizes electronic structure, and facilitates OH − and *OH adsorption. These effects collectively facilitate the activation of the lattice oxygen oxidation mechanism (LOM), as evidenced by mechanistic studies. Furthermore, the catalyst exhibits excellent chloride repellency due to the chromate re‐adsorption layer, unraveled by the obvious negative shifts of point of zero charge (PZC) and open‐circuit potential (OCP) with increasing KOH concentration. Consequently, the CuNiCoFeCrOH catalyst delivers superior OER performance with low overpotentials of 209.2 and 221.2 mV at 10 mA cm −2 in alkaline freshwater and simulated seawater, respectively, along with excellent stability (100 h @200 mA cm −2 ). This work provides valuable insights for the rational design and fabrication of highly active and corrosion‐resistant electrocatalysts tailored for water and seawater oxidation applications.