Probing the Mechanism of Cation Enhanced CO2 Reduction in Acidic Media: Water Structure Determining the Hydrogenation Kinetics

Abstract Tuning the properties of the electric double layer via cations is an efficient and well-accepted approach for improving the activity and selectivity of CO2 reduction reaction (CO2RR). However, the mechanism for cation enhanced CO2RR kinetics is a long-standing puzzle. Here, we identify the key intermediate, i.e., adsorbed CO2, by in-situ attenuated total reflection surface-enhanced infrared absorption spectroscopy, and confirm that the adsorbed CO2 can only be observed in the presence of the cations. Different from the prevalent viewpoints, time-resolved infrared spectra reveal that Li+ prefers to promote the adsorption of CO2 rather than other larger cations, but it slows down the hydrogenation kinetics of CO2. The ab-initio molecular dynamics simulations and the spectroscopic features of water suggest that the rigid water networks around Li+ impedes the hydrogen of water to approach the oxygen of the adsorbed CO2. In contrast, more flexible water networks around larger cations (e.g., Na+) benefit the reorientation of water and increase the possibility of hydrogen approaching CO2, which boosts CO2RR, consequently. This study highlights the essential role of interfacial water structure in enhancing the activity of CO2RR.