Ammonium ionic liquid cation promotes electrochemical CO2reduction to ethylene over formate while inhibiting the hydrogen evolution on a copper electrodeElectronic supplementary information (ESI) available. See DOI: 10.1039/d1cy01584b

Reduction in the cost of renewable electricity has enhanced the viability of the electrochemical CO2reduction reaction (CO2RR) to chemicals. Ethylene is an economically desired product, and Cu is the only cathode that produces C2H4at reasonable faradaic efficiencies. Altering the binding strength of the key intermediate (CO2−) to favor the reaction pathway to ethylene offers an opportunity to enhance its selectivity further. We explore the influence of ionic liquid cations on ethylene/CO2RR and hydrogen evolution reaction (HER) activities on polycrystalline Cu. Alkylated imidazolium, pyrazolium, pyrrolidinium, and ammonium tetrafluoroborates were chosen because of their range of Bader charges on their N atom(s) and pKavalues. Among all cations, the tetraethylammonium cation with moderate Bader charge on N and high pKaof hydration showed the highest ethylene/CO2RR and lowest HER activities, respectively. From density functional theory calculations, it is concluded that the moderate stabilization of the critical intermediate (*COO−) and the decrease in hydrogen binding energy are the reasons for the enhancement of ethylene/CO2RR and suppression of HER activities, respectively.