Seawater electrolysis is an effective way to obtain hydrogen (H₂) in a sustainable manner. However, the lack of electrocatalysts with high activity, stability, and selectivity for oxygen evolution reaction (OER) severely hinders the development of seawater electrolysis technology. Herein, sulfur-doped nickel-iron selenide nanosheets (S-NiFeSe₂) were prepared by an ion-exchange strategy and served as highly active OER electrocatalyst for alkaline seawater electrolysis. The overpotential is 367 mV, and it can run stably for over 50 h at 100 mA cm⁻². Excitingly, the S-NiFeSe₂∥Pt/C pair exhibits cell voltage of 1.54 V at 10 mA cm⁻² under alkaline seawater conditions, which can run smoothly for 100 h without decay, and the efficiency of electricity-to-hydrogen (ETH) energy conversion reaches more than 80%. Such electrode, with abundant accessible reactive sites and good corrosion resistance, is a good candidate for seawater electrolysis. Moreover, density functional theory calculations reveal that the surface sulfur atoms can activate the adjacent Ni sites and decrease the free energy changes of the associated intermediates at the adjacent Ni sites for OER, and the step of *OH → *O is the potential rate-limiting step. In this work, the true reactive site in nickel-iron selenides is the Ni sites, but not the Fe sites as commonly believed.

海水电解是以可持续方式获取氢气（H ₂ ）的有效方法。然而，缺乏具有高活性、稳定性和选择性的析氧反应（OER）电催化剂严重阻碍了海水电解技术的发展。在此，通过离子交换策略制备了硫掺杂的镍铁硒化物纳米片（S-NiFeSe ₂ ），并将其用作碱性海水电解的高活性OER电催化剂。过电位为367 mV，在100 mA cm下可稳定运行50 h以上 ⁻² 。令人兴奋的是，S-NiFeSe ₂ ∥Pt/C 对在碱性海水条件下在 10 mA cm ⁻² 下表现出 1.54 V 的电池电压，可以平稳运行 100 h 而不会衰减，电转氢（ETH）能量转换效率达到80%以上。这种电极具有丰富的可接触反应位点和良好的耐腐蚀性，是海水电解的良好候选者。此外，密度泛函理论计算表明，表面硫原子可以激活相邻的Ni位点并减少OER中相邻Ni位点上相关中间体的自由能变化，*OH→*O的步骤是潜在的速率-限制步骤。在这项工作中，镍铁硒化物中真正的反应位点是 Ni 位点，而不是通常认为的 Fe 位点。