Vitamin C-induced CO capture enables high-rate ethylene production in CO electroreduction.

High-rate production of multicarbon chemicals via the electrochemical CO reduction can be achieved by efficient CO mass transport. A key challenge for C-C coupling in high-current-density CO reduction is how to promote *CO formation and dimerization. Here, we report molecularly enhanced CO-to-*CO conversion and *CO dimerization for high-rate ethylene production. Nanoconfinement of ascorbic acid by graphene quantum dots enables immobilization and redox reversibility of ascorbic acid in heterogeneous electrocatalysts. Cu nanowire with ascorbic acid nanoconfined by graphene quantum dots (cAA-CuNW) demonstrates high-rate ethylene production with a Faradaic efficiency of 60.7% and a partial current density of 539 mA/cm, a 2.9-fold improvement over that of pristine CuNW. Furthermore, under low CO ratio of 33%, cAA-CuNW still exhibits efficient ethylene production with a Faradaic efficiency of 41.8%. We find that cAA-CuNW increases *CO coverage and optimizes the *CO binding mode ensemble between atop and bridge for efficient C-C coupling. A mechanistic study reveals that ascorbic acid can facilitate *CO formation and dimerization by favorable electron and proton transfer with strong hydrogen bonding.