Revealing Structural Evolution of Single Atom Catalysts during Electrochemical CO₂ Reduction by in Situ X-ray Absorption Spectroscopy

Investigating the structural variation of single-atom catalysts (SACs) is crucial to reveal the reaction mechanism under working conditions. Different in situ techniques, especially X-ray absorption spectroscopy (XAS), have been reported to study the structural changes in active sites. However, a systematic study of the relationship between the coordination environment and the catalytic ability of different kinds of SACs is still lacking. Herein, we established isolated transition metal atoms (Fe, Co, Ni, Cu) on N-doped carbon (M–N–C) and employed them in electrocatalytic CO2 reduction reaction (CO2RR). Significantly, Ni–N–C exhibits the highest selectivity (∼97.9%) toward CO at −0.8 V vs RHE. In situ XAS characterization discloses the coordination number dependent catalytic performance. The decreased average coordination number of Ni in Ni–N–C at the voltage point with maximum Faradaic efficiency was observed. Density functional theory further provides the possible mechanism of CO2-to-CO over the undercoordinated Ni–N–C structure.