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 CO2δ− affects electrode-mediated reactions in two distinct modes with opposite effects, i.e., a general inhibitory effect of CO2δ− 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.