Influence of tetraethylammonium cation on electrochemical CO2 reduction over Cu, Ag, Ni, and Fe surfaces

Enhancing the selectivity and activity of desired CO2 reduction products while suppressing hydrogen evolution reaction (HER) will facilitate the commercialization of electro-reduction of CO2 to hydrocarbons. Except for gold, polycrystalline surfaces of all transition metals used for the electrochemical CO2 reduction reaction (CO2RR) suffer from low CO2RR selectivities and activities at lower overpotentials. As ionic liquid cations stabilize key CO2RR intermediates and destabilize the HER intermediates, we studied the influence of tetraethylammonium (TEA+) cation on CO2RR and HER activities, experimentally, over transition metal electrodes (Cu, Fe, Ag, and Ni). We observed an enhancement in CO2RR activity and major CO2RR product formation in the presence of TEA+ cation on Cu and Ag. In addition, HER activity was enhanced on Ni and Fe surfaces but suppressed on Ag and Cu surfaces. Density functional theory (DFT) based computational studies were employed to rationalize these observations. Experimental and computational results suggest that the decreased activation energy for water dissociation enhanced the HER activity on Ni and Fe. Further, the increased activation energy for water dissociation suppressed HER activity on Cu and Ag. Moreover, the increase in *COO– (or CO2–⋅) binding energy in the presence of TEA+ cation is crucial for enhancing CO2RR activity and major CO2RR product formation on Cu and Ag. Thus, this study of the influence of TEA+ cation on CO2RR/HER activity over transition metal electrodes helps in exploring other ionic liquid cation-electrode combinations in terms of CO2–⋅ intermediate stabilization and water dissociation suppression for selective electrochemical CO2 reduction to fuels.