Synthesis of a ruthenium–graphene quantum dot–graphene hybrid as a promising single-atom catalyst for electrochemical nitrogen reduction with ultrahigh yield rate and selectivity

Abstract

It is impossible for the hybrid of classical graphene and metal nanoparticles to break through the limitations of their inherent properties because graphene and metal nanoparticles are conductors. This study reports the synthesis of single atom ruthenium–histidine-functionalized graphene quantum dot–graphene hybrid (Ru–His–GQD–G). His–GQD is immobilized on graphene oxide (GO) via π–π stacking and then coordinated with Ru³⁺ ions. This is followed by reduction with ascorbic acid into GO gel and annealing at 400 °C at a slow heating rate of 0.5 °C min⁻¹. The resulting Ru–His–GQD–G shows a well-defined three-dimensional structure with a high Ru loading of 5.1 wt%, in which Ru atoms are evenly dispersed on graphene sheets. The intimate contact between His–GQD and Ru atom and graphene creates double Schottky heterojunctions. Ru–His–GQD–G exhibits ultrahigh catalytic activity for electrochemical nitrogen reduction. At a low potential of −0.05 V, the NH₃ yield rate reaches 226 μg mg⁻¹ h⁻¹ with the faradaic efficiency of 42.6%, which is better than that of reported electrocatalysts for nitrogen reduction. The experimental and theoretical investigations demonstrate that the Ru sites with His–GQD are the major active centers that permit nitrogen adsorption, stabilization of *NNH and destabilization of *H. This study also provides a way for the construction of graphene-based single-atom catalysts with excellent catalytic activity.