Deciphering role of dual nickel sites in reconstructed hetero-anionic structure of nickel sulfide for boosted hydrogen evolution reaction

The profound understanding of catalyst surface evolution under hydrogen evolution conditions is paramount for precisely regulating active sites, yet it has been somewhat overlooked. In this study, we unveil an innovative spontaneous anion regulation mechanism of nickel sulfide electrocatalysts during the alkaline hydrogen evolution reaction (HER) and elucidate the role of concomitantly generated dual nickel sites. An in-situ reconstructed hetero-anionic nickel oxysulfide surface layer is revealed as the true active sites, tailoring the electronic structure of sulfide-modified nickel sites to promote *H2O dissociation. Simultaneously, the newly introduced oxygen-modified nickel sites effectively serve as *H adsorbers, triggering reversible hydrogen spillover to facilitate *OH transfer and alleviate OH blockage. Therefore, an electrocatalyst-based anion exchange membrane electrolyzer demonstrates remarkable efficiency at industrial current densities, with sustainability maintained at 80 % over 200 h. This study inspires fresh insights into the design of advanced multi-anion catalysts.