Synergistic Multisites Fe₂Mo₆S₈ Electrocatalysts for Ambient Nitrogen Conversion to Ammonia.

Abstract

Electrochemical hydrogenation of N₂ under ambient conditions is attractive for sustainable and distributable NH₃ production but is limited by the lack of selective electrocatalysts. Herein, we describe active site motifs based on the Chevrel phase chalcogenide Fe₂Mo₆S₈ that exhibit intrinsic activities for converting N₂ to NH₃ in aqueous electrolytes. Despite having a very low specific surface area of ∼2 m²/g, this catalyst exhibited a Faradaic efficiency of 12.5% and an average rate of 70 μg h^-1 mg_cat^-1 for NH₃ production at -0.20 V vs RHE. Such activities were attributed to the unique composition and structure of Fe₂Mo₆S₈ that provide synergistic multisites for activating and associating key reaction intermediates. Specifically, Fe/Mo sites assist adsorption and activation of N₂, whereas S sites stabilize hydrogen intermediate H_ad* for N₂ hydrogenation. Fe in Fe₂Mo₆S₈ enhances binding of S with H_ad* and thus inhibits the competing hydrogen evolution reaction. The spatial geometry of Fe, Mo, and S sites in Fe₂Mo₆S₈ promotes conversion of N₂-H_ad* association intermediates, reaching a turnover frequency of ∼0.23 s^-1 for NH₃ production.