Ammonium ionic liquid cation promotes electrochemical CO2 reduction to ethylene over formate while inhibiting the hydrogen evolution on a copper electrode

Abstract

Reduction in the cost of renewable electricity has enhanced the viability of the electrochemical CO₂ reduction reaction (CO₂RR) to chemicals. Ethylene is an economically desired product, and Cu is the only cathode that produces C₂H₄ at 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 pK_a values. Among all cations, the tetraethylammonium cation with moderate Bader charge on N and high pK_a of 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.