Selective Etching Quaternary MAX Phase toward Single Atom Copper Immobilized MXene (Ti₃C₂Clx) for Efficient CO₂ Electroreduction to Methanol

Single atom catalysts possess attractive electrocatalytic activities for various chemical reactions owing to their favorable geometric and electronic structures compared to the bulk counterparts. Herein, we demonstrate an efficient approach to producing single atom copper immobilized MXene for electrocatalytic CO₂ reduction to methanol via selective etching of hybrid A layers (Al and Cu) in quaternary MAX phases (Ti₃(Al1-xCux)C₂) due to the different saturated vapor pressures of Al- and Cu-containing products. After selective etching of Al in the hybrid A layers, Cu atoms are well-preserved and simultaneously immobilized onto the resultant MXene with dominant surface functional group (Clx) on the outmost Ti layers (denoted as Ti₃C₂Clx) via Cu-O bonds. Consequently, the as-prepared single atom Cu catalyst exhibits a high Faradaic efficiency value of 59.1% to produce CH₃OH and shows good electrocatalytic stability. On the basis of synchrotron-based X-ray absorption spectroscopy analysis and density functional theory calculations, the single atom Cu with unsaturated electronic structure (Cuδ+, 0 < δ < 2) delivers a low energy barrier for the rate-determining step (conversion of HCOOH* to absorbed CHO* intermediate), which is responsible for the efficient electrocatalytic CO₂ reduction to CH₃OH.