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Seawater electrolysis is an attractive technique for mass production of high‐purity hydrogen considering the abundance of seawater. Nevertheless, due to the complexity of seawater environment, efficient anode catalyst, that should be, cost effective, highly active for oxygen evolution reaction (OER) but negligible for Cl2/ClO– formation, and robust toward chlorine corrosion, is urgently demanded for large‐scale application. Although catalysis typically appears at surface, while the bulk properties and morphology structure also have a significant impact on the performance, thus requiring a systematic optimization. Herein, a multiscale engineering approach toward the development of cost‐effective and robust OER electrocatalyst for operation in seawater is reported. Specifically, the engineering of hollow‐sphere structure can facilitate the removal of gas product, while atom‐level synergy between Co and Fe can promote Co sites transforming to active phase, and in situ transformation of sulfate ions layer protects catalysts from corrosion. As a result, the as‐developed hollow‐sphere structured CoFeSx electrocatalyst can stably operate at a high current density of 100 mA cm–2 in the alkaline simulated seawater (pH = 13) for 700 h and in a neutral seawater for 20 h without attenuation. It provides a new strategy for the development of electrocatalysts with a broader application potential. [ABSTRACT FROM AUTHOR]

Seawater electrolysis is an attractive technique for massive green hydrogen production owing to the dominant advantages of seawater resources, namely low‐cost and limitlessness. However, the oxygen evolution reaction (OER) catalysts will be easily deactivated for severe seawater Cl− permeation and corrosion. Herein, a structural buffer engineering strategy is reported to endow the Co2(OH)3Cl with long‐term operation stability and a high OER selectivity of ≈99.6% in seawater splitting. The lattice Cl− of Co2(OH)3Cl can act as the structural buffer, whose continuous leaching during OER can leave vacancies for seawater Cl− invasion, so as to avoid catalyst deactivation. Accordingly, Co2(OH)3Cl can maintain 99.9% of its initial current density after 60 000 s operation, while that of Co(OH)2 decays by 52.7% in 7 000 s. Meanwhile, the lattice Cl− of Co2(OH)3Cl can optimize the binding energy of reaction intermediates on the neighboring OCoO site. Thus, Co2(OH)3Cl exhibits a current density of 330.5 mA cm–2 at the potential of 1.63 V versus RHE, 45.9 times higher than that of Co(OH)2. The structural buffer strategy may be applied to incorporate other metal oxides with suitable anions, and effectively boost their OER activity and stability in alkaline seawater. [ABSTRACT FROM AUTHOR]