This study investigates the rate-determining step (RDS) in CO₂ electroreduction to ethylene on different copper (Cu) facets, specifically Cu(100) and Cu(111). The research reveals that the RDS varies depending on the Cu surface, with C–C bond formation being the RDS on Cu(100) and protonation of *CO with adsorbed water being the RDS on Cu(111). Using an oxide-derived Cu(100)-dominant catalyst, the study achieves a high Faradaic efficiency (72%) for ethylene production, a partial current density of 359 mA cm⁻², and long-term stability exceeding 100 hours at 500 mA cm⁻². The catalyst also demonstrates consistent ethylene selectivity of over 60% for 70 hours in a membrane electrode assembly electrolyzer with a full-cell energy efficiency of 23.4%. The results highlight the importance of facet-dependent RDS in determining the performance of Cu catalysts for CO₂ electroreduction. The study combines experimental and computational methods to elucidate the reaction mechanisms and provides insights into designing more efficient Cu-based catalysts for ethylene production via CO₂ electroreduction.This study investigates the rate-determining step (RDS) in CO₂ electroreduction to ethylene on different copper (Cu) facets, specifically Cu(100) and Cu(111). The research reveals that the RDS varies depending on the Cu surface, with C–C bond formation being the RDS on Cu(100) and protonation of *CO with adsorbed water being the RDS on Cu(111). Using an oxide-derived Cu(100)-dominant catalyst, the study achieves a high Faradaic efficiency (72%) for ethylene production, a partial current density of 359 mA cm⁻², and long-term stability exceeding 100 hours at 500 mA cm⁻². The catalyst also demonstrates consistent ethylene selectivity of over 60% for 70 hours in a membrane electrode assembly electrolyzer with a full-cell energy efficiency of 23.4%. The results highlight the importance of facet-dependent RDS in determining the performance of Cu catalysts for CO₂ electroreduction. The study combines experimental and computational methods to elucidate the reaction mechanisms and provides insights into designing more efficient Cu-based catalysts for ethylene production via CO₂ electroreduction.