Electrochemical conversion of CO₂ into valuable chemicals (such as CO) represents an important way to achieve carbon neutrality. In the field of electrocatalysis, atomically isolated metal catalysts (AIMCs) have attracted great attention because of their maximum atom utilization efficiency and tunable coordination microenvironment.  

At present, it remains a challenge to obtain porous carbon-based AIMCs by pyrolyzing precursors (such as polymers, metal-organic framework), as the single-atom metal active species are prone to migrate and agglomerate to form metal nanoparticles (MNPs) during preparation and catalytic processes. 

In a study published in ACS Material Letters, the research team led by Prof. CAO Rong and Prof. HUANG Yuanbiao from Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences developed an effective spatial sites separation strategy (SSSS) to prepare nickel single-atom catalyst (Ni₅-PTF-1000) via separating potential target metal species by intrinsic frameworks.  

The researchers found that the nickel metal sites are anchored by the porphyrin-based covalent triazine framework (CTF), and the target nickel species are effectively separated spatially by using mixed monomers, so that they are captured by the empty nitrogen atoms during the pyrolysis process, thereby obtaining Single atom nickel catalyst.  

To be specific, the researchers copolymerized the TPPCN and Ni-TPPCN (Zn/Ni = 95/5) to give the porous porphyrinic triazine frameworks (denoted as PTF-ZnNi₅) in the presence of ZnCl₂ at 400 ^oC. 

On one hand, they separated the target Ni centers in PTF-ZnNi₅ by the in situ formed Zn-N₄ motifs that originated from the reaction of TPPCN unit and the molten ZnCl₂ during the pyrolysis process. On the other hand, the overflowed Ni species can be stabilized by coordination with the generated free N sites left by zinc species during pyrolysis. This method avoided cumbersome and possibly damaging monoatomic catalysts. 

The Ni₅-PTF-1000 possessed the capability of CO₂ electroreduction towards CO and had a high FE of 94% at -0.9 V in CO₂-saturated 0.5 M KHCO₃. Notably, Ni₅-PTF-1000 exhibited FE_CO over 91% in a wide potential range of -0.6 to -1.0 V. The ATR-IR spectra of Ni₅-PTF-1000 showed the vibration peaks corresponding to C-O stretching of ^*COOH (~1400 cm^–1), which indicated that the formation intermediate of *COOH was the rate determining step for CO₂RR. 

This study develops a facile spatial sites separation strategy to afford atomically isolated nickel species anchored in porous nitrogen-doped carbons as electrocatalysts with high efficiency for CO₂RR. This strategy presents a facile way to develop fully atomically isolated metal catalysts for applications including energy conversion and storage.  

Spatial Sites Separation Strategy to prepare atomically isolated nickel catalyst of Ni₅-PTF-1000 and efficient CO₂ electroreduction (Image by Prof. CAO’s group)