▎ 摘　　要

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 NH3 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 NH3 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 NH3 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 NH3 formation rate exceeding 7.3 mu g h(-1) mg(-1) was achieved at low overpotentials in both acidic and alkaline environments, comparable to the values shown by metal electro-catalysts under similar conditions.