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.