LAUSR

The actual chemical environment of metal phosphides or chalcogenides during hydrogen evolution reaction (HER) has been debated. In this work, pyrite-phase NiP2, Se-doped NiP2, and NiSe2 were investigated by means of in situ and ex situ characterizations to dynamically study the transformation during electrocatalysis. We found that doping with selenide would generate more vacancies to facilitate the activation on water molecule, in which hydroxide poisoning on active site would be suppressed by the presence of selenide. Furthermore, the radial distance of Ni–Ni pair can serve as a descriptor to identify the mechanism of HER. For NiP2 and Se-doped NiP2, the radial distance of the metal (Ni–Ni) pair during HER is identical to the as-prepared one, implying that the HER active site is on a single metal site with a Heyrovsky step. A drastic bond contraction of Ni–Ni pair on NiSe2 indicates dual-site activation correlated with a chemical desorption of Tafel step. The role of anion effects and metal sites are com...