Selectivity and activity trends of single-phase copper-tin foam electrocatalysts in CO2 electroreduction reaction

•Enhanced selectivity for both CO and HCOO− correlates with HER inhibition.•Selectivity for CO production is due to facile formation of *COOH intermediate.•Reciprocal relationship between selectivity and activity for HCOO− is observed. The quest for active, selective, and stable electrocatalysts to convert CO2 into valuable products like carbon monoxide and formate has garnered significant attention in recent years, driven by both fundamental research and practical applications. Recent findings on copper-tin electrocatalysts reveal an intriguing shift in selectivity of CO2 reduction − from producing CO at low Sn concentrations to generating formate with nearly unity selectivity when the Sn content is increased. This shift raises important questions about the factors influencing the dramatic changes in CO2 reduction product distribution as the Cu-Sn material composition varies. However, existing experimental data primarily derive from multiphase Cu-Sn materials, which typically undergo phase changes under CO2 reduction conditions, which introduces interpretation uncertainties. In this study, we developed stable single-phase Cu-Sn materials, specifically a tin solid solution in Cu with the composition of Cu97Sn3 and the intermetallic Cu6Sn5, which were fabricated as dispersed foams to facilitate kinetic measurements. Our findings indicate that the high activity and selectivity of the Cu-Sn solid solution in the CO2-to-CO conversion process are likely due to more favorable kinetics for the formation of the *COOH intermediate, and not due to easier carbon monoxide desorption, as was previously suggested. In contrast, the formate production kinetics for the HCOO-selective Cu6Sn5 phase are significantly inhibited compared to pure copper. We hope our results will motivate further investigation into the nature of the active sites in Cu-Sn electrocatalysts, providing a deeper mechanistic understanding of the observed selectivity/activity trends.