Correlating hydration free energy and specific adsorption of alkali metal cations during CO₂ electroreduction on Au

Specifically adsorbed alkali metal cations on metal electrodes have been hypothesized to influence the reduction of CO₂. However, experimental detection of these cations during CO₂ reduction remains elusive. Herein, employing the asymmetric CH₃ deformation band of tetramethylammonium as a vibrational probe of the aqueous electrolyte–polycrystalline Au interface, we monitored the displacement of specifically adsorbed tetramethylammonium by alkali metal cations. We found that the coverage of specifically adsorbed alkali metal cations during CO₂-to-CO reduction follows the order Li⁺ < Na⁺ < K⁺ < Cs⁺ for the same bulk concentration. The alkali metal cations’ experimentally observed surface coverages correlate with their free energies of hydration. Furthermore, the rate of CO₂-to-CO conversion increases with the coverage of specifically adsorbed alkali metal cations. Our observations suggest that the degree to which alkali metal cations undergo partial dehydration at the electrode–electrolyte interface plays a key role in their ability to promote CO₂-to-CO reduction. [Figure not available: see fulltext.]