Evoking the Dynamic Fe-Nx Active Sites through the Immobilization of Molecular Fe Catalyst on N-doped Graphene Quantum Dot for the Efficient Electroreduction of Nitrate to Ammonia

Abstract

The excessive energy demands of the conventional Haber-Bosch process for ammonia (NH3) generation, coupled with the disruptive effects of nitrate (NO3‒) pollution on the global nitrogen cycle, had drawn the electrocatalytic nitrate reduction reaction (NO3‒RR) as an essential exit strategy for a sustainable NH3 synthesis. However, the intricate multi-step proton and electron transfer process posed a significant challenge in achieving high-efficiency electrocatalyst. Herein, in this study, we reported a selective and highly active NO3‒RR electrocatalyst featuring molecular M‒Nx sites derived from the immobilization of the Fe ions within the N-doped graphene quantum dots (NGQDs). We demonstrated the formation of molecular Fe‒Nx coordination activated the NO3‒RR activity of the NGQDs-Fe, despite the initial inactivity of NGQDs. The in-situ Raman analysis revealed that those Fe‒Nx sites served as favorable adsorption sites for *NO3. Such catalyst achieved 93% NH3 FE and a yield rate of 15.41 mmol/h.cm2 efficiency at ‒0.8 V (vs. RHE) in an alkaline medium. These findings unveiled the preferential sequential 2e− and 6e− transfer pathway over the direct 8e− pathway in NO3‒RR, which provides new mechanistic insight into the nitrate reduction reaction.