Introducing sulfur to nickel-iron selenide for high-efficiency alkaline seawater electrolysis

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.