(517c) Enhanced Electrocatalytic CO2 Reduction on Thiol-Functionalized Gold

CO₂ can potentially be used as a renewable feedstock for distributed electrochemical production of fuels and chemicals, either as an energy carrier to store energy generated from intermittent sources such as wind and the sun, or as a renewable source of carbon for chemical production. The key to enabling these uses of CO₂ is maximizing the selectivity and energy efficiency of electrocatalytic CO₂ reduction. Hori and co-workers have shown that some metals (Au, Ag and Cu, etc.) can catalyze the reaction and produce CO or hydrocarbons [Hori, in Modern Aspects of Electrochemistry, ed. Vayenas et al., Springer (2008)]. A significant challenge is posed by the competing hydrogen evolution reaction (HER), which tends to occur with a smaller overpotential and consumes a significant fraction of the current and protons. Furthermore, Nørskov and co-workers have shown that the catalytic activity for CO₂ electroreduction is limited by the strong scaling relation that exists between the key reaction intermediate, COOH, and the reaction product, CO [Shi et al., Phys. Chem. Chem. Phys. 16 (2014) 4720]. In present work, we take inspiration from biological enzymes that catalyze CO₂ reduction (e.g. CO dehydrogenase) and attempt to mimic their active sites by functionalizing Au electrodes with a series of organic thiol-based ligands, to seek to potentially overcome these challenges. This has resulted in significantly altered activity for CO formation and selectivity between CO and H₂. The CO yield was increased dramatically and nearly doubled in the presence of 2-phenylethanethiol (2-PET) compared to the blank gold foil electrode. On the contrary, 2-merceptanpropanic acid (MPA) suppresses the yield of CO to negligible amounts. The 2-PET-modified gold electrode also reduced the faradaic efficiency for HER by half, whereas the MPA-modified Au electrode increased it. Density function theory based modeling suggest that ligand-induced surface reconstruction of the Au surface is a major factor in the altered catalytic activity compared to blank Au, which creates sites that favor CO production over HER. Moreover, the surface-bound thiol ligands modify the adsorption energies of surface reduction intermediates through electronic interaction and further contribute to the altered activities for CO₂ reduction and HER.