Valence engineering at the interface of MoS2/Mo2C heterostructure for bionic nitrogen reduction

•We successfully synthesize a two-dimensional MoS2/Mo2C heterostructure, with finely tuned and a broad valence spectrum.•A two-dimensional MoS2/Mo2C heterostructure shows high performance for electro-synthesis of ammonia.•DFT reveals the reaction mechanism and the specific active center Mo3+ site at the MoS2/Mo2C hetero-boundary. The natural nitrogenase is still the most efficient catalyst on earth to reduce the ambient N2 into ammonia. The central part of the molecular machine is powered by a metallic core, usually a molybdenum atom, whose coordination valence state remains an enigma for us to unveil and mimic. Unlike the flexible bio-enzyme, inorganic heterogeneous catalysts are usually rigid in the coordination structure, making their valence states invariable, except some localized defects. In this study, we successfully synthesized a two-dimensional MoS2/Mo2C electrocatalyst, which contains a heterostructured interface with efficient charge and magnetism separation, exhibiting a gradual and broad valence state transition from Mo4+ to Mo2+. Density functional theory (DFT) calculations reveal that Mo3+ sites at the interface have a strong N2 adsorption energy of − 0.75 eV with the side-on configuration, and an activated hydrogenation of *NH2 species. This bionic electrocatalyst displays a splendid performance in nitrogen reduction reaction with a Faradic efficiency of 42 % at −0.1 V vs RHE.