Approaching 100% Selectivity at Low Potential on Ag for Electrochemical CO₂ Reduction to CO Using a Surface Additive

Abstract

We report the discovery of a quaternary ammonium surface additive for CO₂ reduction on Ag surfaces that changes the Faradaic efficiency for CO from 25% on Ag foil to 97%, while increasing the current density for CO production by a factor of 9 from 0.14 to 1.21 mA/cm² and reducing the current density for H₂ production by a factor of 440 from 0.44 to 0.001 mA/cm². Using ReaxFF reactive molecular dynamics, we find that the surface additive with the highest selectivity, dihexadecyldimethylammonium bromide, promotes substantial population of CO₂ near the Ag surface along with sufficient H₂O to activate the CO₂. While a critical number of water molecules is required in the reduction of CO₂ to CO, the trend in selectivity strongly correlates with the availability of CO₂ molecules. We demonstrate that the ordering of the cationic modifiers plays a significant role around the active site, thus determining reaction selectivity. The dramatic improvement by addition of a simple surface additive suggests an additional strategy in electrocatalysis.