Reaction-driven formation of anisotropic strains in FeTeSe nanosheets boosts low-concentration nitrate reduction to ammonia

FeM (M = Se, Te) chalcogenides have been well studied as promising magnets and superconductors, yet their potential as electrocatalysts is often considered limited due to anion dissolution and oxidation during electrochemical reactions. Here, we show that by using two-dimensional (2D) FeTeSe nanosheets, these conventionally perceived limitations can be leveraged to enable the reaction-driven in-situ generation of anisotropic in-plane tensile and out-of-plane compressive strains during the alkaline low-concentration nitrate reduction reaction (NO₃^-RR). The reconstructed catalyst demonstrates enhanced performance, yielding ammonia with a near-unity Faradaic efficiency and a high yield rate of 42.14 ± 2.06 mg h^-1 mg_cat^-1. A series of operando synchrotron-based X-ray measurements and ex-situ characterizations, alongside theoretical calculations, reveal that strain formation is ascribed to chalcogen vacancies created by partial Se/Te leaching, which facilitate the adsorption and dissociation of OH^-/NO₃^- from the electrolyte, resulting in an O(H)-doped strained lattice. Combined electrochemical and computational investigations suggest that the superior catalytic performance arises from the synergistic contributions from the exposed strained Fe sites and surface hydroxyl groups. These findings highlight the potential of 2D transition metal chalcogenides for in-situ structural engineering during electrochemical reactions to enhance catalytic activity for NO₃^-RR and beyond.