Boosting the Electroreduction of CO₂ to CO by Ligand Engineering of Gold Nanoclusters

The electrochemical CO₂ reduction reaction (CO₂RR) has been widely studied as a promising means to convert anthropogenic CO₂ into valuable chemicals and fuels. In this process, the alkali metal ions present in the electrolyte are known to significantly influence the CO₂RR activity and selectivity. In this study, we report a strategy for preparing efficient electrocatalysts by introducing a cation-relaying ligand, namely 6-mercaptohexanoic acid (MHA), into atom-precise Au₂₅ nanoclusters (NCs). The CO₂RR activity of the synthesized Au₂₅(MHA)₁₈ NCs was compared with that of Au₂₅(HT)₁₈ NCs (HT=1-hexanethiolate). While both NCs selectively produced CO over H₂, the CO₂-to-CO conversion activity of the Au₂₅(MHA)₁₈ NCs was significantly higher than that of the Au₂₅(HT)₁₈ NCs when the catholyte pH was higher than the pK_a of MHA, demonstrating the cation-relaying effect of the anionic terminal group. Mechanistic investigations into the CO₂RR occurring on the Au₂₅ NCs in the presence of different catholyte cations and concentrations revealed that the CO₂-to-CO conversion activities of these Au₂₅ NCs increased in the order Li⁺<Na⁺<K⁺<Cs⁺, and are gated by the cation-coupled electron transfer step. These results were confirmed by the Nernstian shifts of the polarization curves at different cation concentrations.