Multifunctional Molecular Cages Boost Acidic CO₂ Electroreduction to Ethylene

Acidic electrocatalytic CO₂ reduction (CO₂RR) to multicarbon (C₂₊) products offers a promising pathway for efficient carbon utilization, as acidic media suppress carbonate formation and improve CO₂ availability. However, two intrinsic limitations, rapid reduction of Cu⁺ to Cu⁰ and fast desorption/diffusion of the *CO intermediate, severely hindered C-C coupling and thus diminish ethylene (C₂H₄) selectivity in acidic CO₂RR. Here, we constructed a molecular cage on the Cu₂O surface by grafting cetyltrimethylammonium bromide (CTAB), which simultaneously stabilized Cu⁺ and restricted *CO diffusion, thereby enabling efficient CO₂-to-C₂H₄ conversion under strongly acidic conditions. Density functional theory calculations revealed that the molecular cage significantly increased local *CO coverage, which lowered the C-C coupling energy barrier while raising the energy barrier for hydrogen evolution reduction. In situ attenuated total reflection infrared spectroscopy demonstrated that CTAB-induced confinement strengthened *CO adsorption and slowed its surface diffusion, accelerating the C-C coupling kinetics. Furthermore, in situ X-ray adsorption near-edge structure confirmed that the molecular cage effectively prevented the reduction of Cu⁺ to metallic Cu, maintaining the active Cu⁺ species during operation. As a result, the optimized Cu₂O@CTAB catalyst delivered a high C₂H₄ Faradaic efficiency of 60% across 300-1100 mA cm^-2 in the strongly acidic electrolyte. Notably, it achieved a CO₂ single-pass utilization of 64.7%, an energy efficiency of 37.9%, and stable operation for over 195 h at 500 mA cm^-2 toward C₂₊ products. This work presents a generalizable molecular-cage strategy for overcoming intrinsic bottlenecks in acidic CO₂RR toward efficient C₂₊ product formation.