Evoking dynamic Fe–Nx active sites through the immobilization of molecular Fe catalysts on N-doped graphene quantum dots for the efficient electroreduction of nitrate to ammonia

Abstract

The excessive energy demand of the conventional Haber–Bosch process for ammonia (NH₃) generation, coupled with the disruptive effects of nitrate (NO₃⁻) pollution on the global nitrogen cycle, has made the electrocatalytic nitrate reduction reaction (NO₃⁻RR) an essential exit strategy for sustainable NH₃ synthesis. However, the intricate multi-step proton and electron transfer process posed a great challenge in achieving high-efficiency electrocatalysts. In this study, we report a selective and highly active NO₃⁻RR electrocatalyst featuring molecular M–N_x sites derived from the immobilization of Fe ions within N-doped graphene quantum dots (NGQDs). We demonstrated that the formation of molecular Fe–N_x coordination activated the NO₃⁻RR of NGQDs-Fe, despite the initial inactivity of NGQDs. In situ Raman analysis revealed that those Fe–N_x sites served as favourable adsorption sites for *NO₃. Such catalyst achieved an FE of 93% and a yield rate of 15.41 mmol h⁻¹ cm⁻² for NH₃ at −0.8 V (vs. RHE) in an alkaline medium. These findings revealed the preferential sequential 2e⁻ and 6e⁻ transfer pathways over the direct 8e⁻ pathway in the NO₃⁻RR, which provides new mechanistic insights into the nitrate reduction reaction.