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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.

Electrochemical carbon dioxide reduction reaction (CO2RR) into chemicals can store renewable electricity and simultaneously control global warming. Albeit inexpensive copper electro-catalyzes CO2to hydrocarbons at reasonable rates, it suffers from low selectivity due to high hydrogen evolution reaction (HER) activity. Although an increase in bulk pH suppresses HER activity, this leads to a CO2reaction with hydroxide and the formation of bicarbonate and carbonate ions. In contrast, we demonstrate selective HER suppression by functionalizing Cu electrocatalysts with pyridine molecules without a commensurate reduction in CO2RR activity. Density functional theory (DFT) based computational results suggest that the decrement in the H binding energy and blockage of step edge atoms (which are the most active sites for the HER) by pyridine molecules are the reasons for selective suppression of the HER activity on the copper surface after pyridine functionalization. Further, an insignificant decrease in CO binding is the reason behind the lack of suppression of the CO2RR activity. This study provides pointers towards better electrocatalyst design for the CO2RR.

Defects on Cu surfaces are created to tune the selectivity of CO2electroreduction toward C2+products. However, mixed defect structures and the co-existence of defect-induced geometric and electronic effects impose a grand challenge to correlate the defect type with product selectivity. Here, we explore the underlying defect parameter determining selectivity.