Enhancing C2+ Product Faradaic Efficiency in CO2 Reduction Using Fluorine-Stabilized Superhydrophobic Copper (δ+)

Abstract

Oxide-derived (OD) Cu catalysts are recognized for their effectiveness in producing C₂₊ products, but often revert to their metallic state, reducing selectivity due to the loss of their positive oxidation state. Here, we report a novel strategy to incorporate fluorine (F) into Cu₂O nanospheres using hydrofluoric acid (HF). While halogen acids have traditionally been employed to etch metals and create cavities for confinement effects, this work goes a step further by inserting F ions into the lattice. Among all halogens, F provided the best lattice stability and was thus selected for catalyst testing. F incorporation was found to stabilize the Cu (δ+) oxidation state, achieving an impressive Faradaic efficiency (FE) of 91.9 ± 2.03% for C₂₊ products, predominantly ethylene (67%), at a current density of 250 mA cm^–2. High-field 1D and 2D ¹⁹F magic-angle spinning (MAS) solid-state nuclear magnetic resonance (ssNMR) spectroscopy provided definitive evidence of F substitution at oxygen vacancies and the formation of a surface layer of HF. Water contact angle (WCA) measurements revealed enhanced hydrophobicity, with the 3 M HF-treated Cu₂O exhibiting superhydrophobicity (WCA = 161°), which effectively suppressed hydrogen evolution (HER). In situ Raman spectroscopy confirmed prolonged stability of the Cu₂O phase in the F-incorporated catalyst, while in situ ATR-FTIR spectroscopy with isotopic labeling elucidated the mechanistic pathway for ethylene production. Additionally, density functional theory (DFT) calculations offered mechanistic insights into ethylene formation, while Bader charge analysis revealed the electronic role of F incorporation in Cu₂O, thereby providing insight into its enhanced selectivity.