Highly exfoliated Ti₃C₂T_{: X} MXene nanosheets atomically doped with Cu for efficient electrochemical CO₂reduction: An experimental and theoretical study

Ti3C2Tx MXene nanostructures have garnered attention for various catalytic applications due to their built-in electronic properties. Herein, we rationally design highly exfoliated two-dimensional Ti3C2Tx nanosheets (Tx = O, OH, and F) doped with Cu (denoted as Cu/Ti3C2Tx) for the electrochemical CO2 reduction reaction (CO2RR). The fabrication process entails the selective chemical etching of Ti3AlC2 followed by the delamination thereof under ultrasonic treatment and subsequent mixing with a Cu precursor to allow in situ doping. The resultant Cu/Ti3C2Tx are highly exfoliated nanosheets with a surface area of 46 m2 g-1 and are uniformly doped with Cu atoms (1.04 wt%). The CO2RR current density of Cu/Ti3C2Tx (-1.08 mA cm-2) was 3.6 times higher than that of Ti3C2Tx (-0.3 mA cm-2) besides a lower onset reduction potential and Tafel slope, and higher stability, due to the greater surface area, electronic effect, and quicker charge transfer on Cu/Ti3C2Tx. The formic acid (HCOOH) faradaic efficiency on Cu/Ti3C2Tx (58.1%) was 3-fold higher than that on Ti3C2Tx (18.7%). Based on density functional theory (DFT) simulation, Cu-doping induces polarized sites with high electron density, allowing the CO2RR path through the ∗HCOOH intermediate to form formic acid (HCOOH). The study presented here will open new pathways for using Ti3C2Tx doped with various metals for the CO2RR.