Selective conversion of CO2 to fuels and chemicals has been considered one of the key challenges in the electrochemical CO2 reduction reaction (CO2RR). Here, we demonstrate the reaction pathways for CO and C2H4 formation on Cu can be regulated by supplying different CO2 partial pressures. Although it is believed high concentration of surface bound CO is required for C2H4 formation, we show excessive supply of CO2 interferes with C–C coupling and suppress C2H4 reaction pathways. This indicates C2H4 reaction pathways are limited by the surface recombination of surface bound CO and hydrogen, and the kinetics is affected by adsorbate-adsorbate interactions and/or by physical blocking of active sites on Cu with excess CO2. Through systematic study, we demonstrate a dilute CO2 stream selectively activates C2H4 formation with significant reduction of the overpotentials (∼ 400 mV) to achieve ∼50% C2H4 Faradaic efficiency and enhancement in the C2H4 current density (∼50 mA cm−2).
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