The electrocatalytic CO₂ reduction reaction (CO₂RR) has the potential to produce fuels and chemicals from a renewable feedstock: It can be used as an energy carrier to store energy generated from intermittent, distributed sources such as wind and the sun. It can also be used as a renewable source of carbon for chemical production. The key to enabling these uses of CO₂ is to maximize the selectivity and energy efficiency of electrocatatlytic CO₂RR toward the desired product. We have taken inspiration from biological enzymes that catalyze CO₂RR (e.g. carbon monoxide dehydrogenase) and attempt to mimic their active sites by functionalizing Au electrodes with a series of organic thiol ligands, to seek opportunities to potentially break the constraints imposed by scaling relations. Our work shows that different thiol species can enhance the activity for the formation of different CO₂ reduction products. For instance, 2-merceptanpropionic acid effectively reduces the yield of CO to negligible amounts and enhances H₂ evolution to nearly 100% faradaic efficiency. 2-phenylethanethiol, on the other hand, strongly promotes CO evolution and suppresses H₂ evolution, whereas 4-pyridinylethanemercaptan boosts the faradaic efficiency toward formate by suppressing CO formation.

We have performed detailed density functional theory calculations and theoretical modeling in conjunction with our electrochemical experiments to elucidate the role of the thiol ligands in CO₂RR on Au. We find that thiol ligands exist primarily as thiolates on Au in the potential range relevant to CO₂RR. They can readily reconstruct Au surfaces at ambient conditions, leading to a higher proportion of under-coordinated sites where the activity of CO₂ reduction is significantly promoted but the effect of under-coordinated Au sites on H₂ evolution is less prominent. In addition, different functional groups on thiols create different local reaction environments on the Au electrode resulting in different product speciation. Spectroscopic evidence for thiol and non-thiol ligands adsorbed on Au will also be discussed; for instance, S-binding and C-binding ligands exhibit distinct vibrational signatures with different electrochemical stabilities. Overall our study suggests that functionalization of Au holds significant promise for promoting CO₂ reduction and achieving different product selectivity with appropriately chosen thiol-based ligands.