Reduced graphene oxides with engineered defects enable efficient electrochemical reduction of dinitrogen to ammonia in wide pH range

Electrochemical nitrogen fixation under mild conditions is highly demanded yet remains a grand challenge. Herein we report metal-free electrocatalysis of aqueous N 2 reduction to produce NH 3 over reduced graphene oxide (DrGO) with tuned defects. The defect sites in DrGO, consisting of unsaturated carbon [single vacancy (SV), double vacancy (DV), and –COOH], were examined, and showed an improved NH 3 selectivity due to the strong binding of N 2 instead of H. In addition to improved selectivity, the calculated free energies for N 2 reduction reaction at DrGO-COOH and DrGO-DV sites suggest that the thermodynamic overpotentials of these metal-free catalysts are comparable to the most efficient transition metal-based catalysts reported thus far. Our nonmetallic and dopant-free catalysts can convert N 2 to NH 3 at a faradaic efficiency of up to 22.0% at −0.116 V (versus the reversible hydrogen electrode vs . RHE) in 0.1 M HCl and 10.8% at −0.166 V ( vs . RHE) in 0.1 M KOH, surpassing most earlier reported catalysts. An NH 3 formation rate exceeding 7.3 μg h −1  mg −1 was achieved at low overpotentials in both acidic and alkaline environments , comparable to the values shown by metal electrocatalysts under similar conditions.