Unraveling the Reactivity and Selectivity of Atomically Isolated Metal-Nitrogen Sites Anchored on Porphyrinic Triazine Frameworks for Electroreduction of CO2

Electroreduction of CO2 (CO2RR) to value-added chemicals offers a promising approach to balance the global carbon emission, but still remains a significant challenge due to high overpotential, low faradaic efficiency, and poor selectivity of electrocatalysts systems. Thus the key point is to develop low-cost, highly efficient, and durable electrocatalysts for CO2RR. To benefit from their exposed active sites and to maximize atomic efficiency, single-metal atom catalysts that usually show high activities are required. Herein, we unravel the trends in the reactivity and selectivity of atomically isolated M-N-4 (M = Ni, Cu, Fe, and Co) sites within porous porphyrinic triazine framework (metal single atoms/PTF) for the electroreduction of CO2 to CO. We found that NiSAs/PTF exhibited the highest faradaic efficiency (98%) at a mild potential of -0.8 V versus reversible hydrogen electrode and the highest turnover frequency of 13,462 h(-1) for the production CO at an applied potential of -1.2 V. The relations of catalytic performance of CO2 to CO over the different active M-N-4 sites were unraveled by the combination of density functional theory calculations and experiments. This work gives an extensive mechanistic understanding of the selectivity of CO2 to CO from the M-N-4 sites at an atomic scale, thus it will bring new inspiration for the design of highly efficient CO2 RR.