Electrochemical reduction of CO₂ (CO₂RR) to highly value-added chemicals and fuels using renewable electricity is a promising strategy to balance the global carbon. However, the low energetic efficiency, sluggish electron transfer kinetic, poor selectivity and low stability of electrode materials remain as significant challenges that impede the practical use and technological commercialization.

Single-atom catalysts (SACs) have attracted broad research interest because of their maximized atom utilization efficiency, highly catalytic activity and recyclability. However, the reactivity and selectivity of different single-atom metal sites still needs to be clear.

In a study published in CCS Chemistry, the research team led by Prof. CAO Rong and Prof. HUANG Yuanbiao from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences unraveled the trends in the reactivity and selectivity of atomically isolated M-N₄ (M = Ni, Cu, Fe, and Co) sites within porous porphyrinic triazine framework (MSAs/PTF) for the electroreduction of CO₂ to CO.

The researchers found that among the four single-atom metal catalysts, Ni/SAs/PTF turned out to be one of the most excellent catalyst, with the highest Faradaic efficiency (98%) for the conversion CO₂ to CO at a mild potential of -0.8 V (vs. the reversible hydrogen electrode, RHE.) and the highest turnover frequency of 13,462 h^-1 at an applied potential of -1.2 V.

The density functional theory (DFT) calculations revealed that the electrochemical limiting potentials for NiSAs/PTF and FeSAs/PTF are lower than those of CuSAs/PTF and CoSAs/PTF. The hydrogen evolution reaction (HER) overpotential for NiSAs/PTF is significant lower than CO₂RR, so Ni SAs/PTF show high selectivity of CO rather than H₂.

This study might shed light on the designing efficient electrocatalysts to address CO₂RR problems.

The structure and CO Faradaic efficiency for MSAs/PTF (M = Ni, Cu, Fe and Co) (Image by Prof. CAO’s group)