Cation-Dependent Impact of CO₂ on Cu-Catalyzed Electrochemical CO Reduction Reaction

The electrochemical CO₂ reduction reaction (CO₂RR) on Cu occurs via two tandem steps, i.e., the CO₂-to-CO conversion and the electrochemical CO reduction reaction (CORR). Thus, Cu-catalyzed CO₂RR on Cu is, in fact, coelectrolysis of CO₂ and CO, which makes the impact of interfacial CO₂ on the CORR a relevant factor in determining the overall CO₂RR performance. In this work, we report the surprising observation that the effect of interfacial CO₂ on the formation rate of C₂₊ products depends on the nature of cations in the electrolyte, i.e., beneficial with K⁺ and Cs⁺ while inhibitory with Li⁺ and Na⁺. Density functional theory calculations indicate that interfacial CO₂^δ− affects electrode-mediated reactions in two distinct modes with opposite effects, i.e., a general inhibitory effect of CO₂^δ− as an interfacial negatively charged species and a cation-specific promoting effect via direct CO₂-cation interactions. The relative contributions of these two competing effects determine the overall impact of CO₂ on the CORR. Furthermore, analysis of isotopologue distributions of products in the coelectrolysis of mixtures of ¹³CO/¹²CO₂ reveals the influence of cations on the multiple steps of the CO₂RR.