Electrochemical Reduction of CO₂ at Functionalized Au Electrodes

Electrochemical reduction of CO₂ provides an opportunity to store renewable energy as fuels with much greater energy densities than batteries. Product selectivity of the reduction reaction is known to be a function of the electrolyte and electrode; however, electrodes modified with functional ligands may offer new methods to control selectivity. Here, we report the electrochemical reduction of CO₂ at functionalized Au surfaces with three thiol-tethered ligands: 2-mercaptopropionic acid, 4-pyridinylethanemercaptan, and cysteamine. Remarkably, Au electrodes modified with 4-pyridinylethanemercaptan show a 2-fold increase in Faradaic efficiency and 3-fold increase in formate production relative to Au foil. Conversely, electrodes with 2-mercaptopropionic acid ligands show nearly 100% Faradaic efficiency toward the hydrogen evolution reaction, while cystemine-modified electrodes show 2-fold increases in both CO and H₂ production. We propose a proton-induced desorption mechanism associated with pK_a of the functionalized ligand as responsible for the dramatic selectivity changes.