Electrocatalytic reduction of CO₂ has the potential to produce fuels and chemicals from renewable feedstocks:  It can be used as an energy carrier (if converted into energy-dense, transportation-ready forms) 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 maximizing the selectivity and energy efficiency of electrocatatlytic CO₂ reduction.  Hori and co-workers have shown that various metals (Au, Ag and Cu, etc.) have the ability to catalyze the reaction and produce CO or hydrocarbons.(1)   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 total current.  Nørskov and co-workers have shown that the catalytic activity for CO₂ electro-reduction is limited by the strong scaling relation that exists between the key reaction intermediate, COOH, and the reaction product, CO.(2)  Furthermore, the catalyst particles would have to be nano-sized in order to maximize the activity of metals such as Cu and Au.(3, 4)

In present work, we take inspiration from biological enzymes that catalyze CO₂ reduction (e.g. carbon monoxide dehydrogenase) and attempt to mimic their active sites by functionalizing Au electrodes with a series of organic thiol-based ligands, to seek to potentially break the constraints imposed by the scaling relations.  This has resulted in significantly altered activity for CO formation and selectivity between CO and H₂.  As shown in the Figure (measured in 0.1 M KHCO₃), 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.  Meanwhile, the 2-PET-modified gold electrode reduced the faradaic efficiency for HER by half, whereas the MPA-modified Au electrode doubled 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.

Reference

1.  Y. Hori, in Modern Aspects of Electrochemistry, C. Vayenas, R. White and M. Gamboa-Aldeco Editors, p. 89, Springer New York (2008).

2.  C. Shi, H. A. Hansen, A. C. Lausche and J. K. Norskov, Physical Chemistry Chemical Physics, 16, 4720 (2014).

3.  D. R. Kauffman, D. Alfonso, C. Matranga, H. Qian and R. Jin, Journal of the American Chemical Society, 134, 10237 (2012).

4.  F. Studt, M. Behrens, E. L. Kunkes, N. Thomas, S. Zander, A. Tarasov, J. Schumann, E. Frei, J. B. Varley, F. Abild-Pedersen, J. K. Nørskov and R. Schlögl, ChemCatChem, 7, 1105 (2015).