Multifunctional Molecular Cages Boost Acidic CO ₂ Electroreduction to Ethylene

Acidic electrocatalytic CO 2 reduction (CO 2 RR) to multicarbon (C 2+ ) products offers a promising pathway for efficient carbon utilization, as acidic media suppress carbonate formation and improve CO 2 availability. However, two intrinsic limitations, rapid reduction of Cu + to Cu 0 and fast desorption/diffusion of the *CO intermediate, severely hindered C–C coupling and thus diminish ethylene (C 2 H 4 ) selectivity in acidic CO 2 RR. Here, we constructed a molecular cage on the Cu 2 O surface by grafting cetyltrimethylammonium bromide (CTAB), which simultaneously stabilized Cu + and restricted *CO diffusion, thereby enabling efficient CO 2 -to-C 2 H 4 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 Cu 2 O@CTAB catalyst delivered a high C 2 H 4 Faradaic efficiency of 60% across 300–1100 mA cm –2 in the strongly acidic electrolyte. Notably, it achieved a CO 2 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 C 2+ products. This work presents a generalizable molecular-cage strategy for overcoming intrinsic bottlenecks in acidic CO 2 RR toward efficient C 2+ product formation.