Dynamic Evolution of Bimetallic Single-Atom Catalysts during Carbon Dioxide Reduction Probed by Operando X‑ray Spectroscopy.

Electrochemical CO2 reduction (CO2RR) offers a promising strategy to convert CO2 into high-value chemicals and fuels. However, probing the mechanism of dynamic catalyst reconstruction remains a longstanding challenge, which calls for operando time-resolved and quantitative spectroscopic methods. Here, we employed operando high-energy-resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) to monitor real-time structural evolution of bimetallic FeCu single-atom catalysts (SACs) on nitrogen-doped carbon (FeCu-N-C) as a model single-atom catalyst for the CO2RR. FeCu-N-C exhibits a 2-fold increase in Faradaic efficiency for CO formation and an order-of-magnitude enhancement in the CO partial current density at the same applied potential, when compared to monometallic Fe SAC (Fe-N-C). To elucidate the origin of this performance enhancement, operando HERFD-XAS was used to directly probe the dynamic evolution of metal oxidation states and coordination environments under operating conditions, revealing a dynamic reconstruction of isolated Fe and Cu single atoms into catalytically active metallic clusters. In contrast, the Fe SAC control remains atomically dispersed over the course of CO2RR. The atomic-scale structural insights are further corroborated by aberration-corrected scanning transmission electron microscopy. These findings shed light on the synergistic role of Cu in modulating the dynamic reconstruction of single-atom Fe sites and provide fundamental insights for the rational design of high-performance single-atom catalysts for CO2 electroreduction.