In Situ and Operando Analytical Techniques of Single-Atom Catalysts for Electrocatalytic CO₂ Reduction

Electrocatalytic technology, which facilitates the transformation of carbon dioxide (CO₂) into high-value chemicals, stands as one of the most hopeful approaches for CO₂ utilization. Single-atom catalysts (SACs) are promising for catalyzing CO₂ reduction reactions (CO₂RR) owing to the tunable electronic structures of their central metal atoms, which enable precise control over the adsorption energies of reactants and intermediates. Additionally, SACs bridge the gap between homogeneous and heterogeneous catalysts, offering an ideal platform to investigate the reaction mechanisms of CO₂RR. Therefore, gaining a comprehensive understanding of the intrinsic structural evolution of SACs, along with the micro-environmental changes around active sites and electrode interfaces under operational conditions, is crucial for designing effective electrocatalysts and devices for CO₂RR. This review introduces the fundamentals underlying the electrocatalytic CO₂RR. Subsequently, the key techniques for SACs identification and validation are thoroughly analyzed, laying a theoretical basis for the case studies. Third, the latest development of in situ and operando analytical techniques of SACs toward CO₂RR are summarized, including infrared spectroscopy (IR), Raman spectroscopy, X-ray absorption spectroscopy (XAS), and transmission electron microscopy (TEM). Finally, several issues are raised and possible solutions are offered regarding the in situ and operando analytical techniques of SACs for the CO₂RR.