Facet-switching of rate-determining step on copper in CO-to-ethylene electroreduction.

Reduction of carbon dioxide (CO) by renewable electricity to produce multicarbon chemicals, such as ethylene (CH), continues to be a challenge because of insufficient Faradaic efficiency, low production rates, and complex mechanistic pathways. Here, we report that the rate-determining steps (RDS) on common copper (Cu) surfaces diverge in CO electroreduction, leading to distinct catalytic performances. Through a combination of experimental and computational studies, we reveal that C─C bond-making is the RDS on Cu(100), whereas the protonation of *CO with adsorbed water becomes rate-limiting on Cu(111) with a higher energy barrier. On an oxide-derived Cu(100)-dominant Cu catalyst, we reach a high CH Faradaic efficiency of 72%, partial current density of 359 mA cm, and long-term stability exceeding 100 h at 500 mA cm, greatly outperforming its Cu(111)-rich counterpart. We further demonstrate constant CH selectivity of >60% over 70 h in a membrane electrode assembly electrolyzer with a full-cell energy efficiency of 23.4%.