Probing electrolyte effects on cation-enhanced CO2 reduction on copper in acidic media

Tuning the properties of the electric double layer via cations is a recognized approach for improving the efficiency of the CO2 reduction reaction (CO2RR). However, the mechanism behind cation-enhanced CO2RR kinetics remains puzzling. Here we identify the key intermediate, adsorbed CO2, via in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy on Cu in an acidic electrolyte, confirming it appears only in the presence of cations. Different from prevalent viewpoints, time-resolved infrared spectra reveal that Li+ enhances CO2 adsorption more effectively than other larger cations but slows down the hydrogenation kinetics of CO2. Ab initio molecular dynamics simulations and spectroscopic features of water suggest that rigid water networks around Li+ impedes the hydrogen of water to approach adsorbed CO2. In contrast, more flexible water networks around larger cations (for example, Na+) facilitate water reorientation and enhance hydrogen proximity to CO2, thereby improving CO2RR. This study highlights the essential role of interfacial water structure in CO2RR efficiency. CO2 electroreduction is promoted by alkali cations in the electrolyte, but the precise mechanism by which this occurs is not clear. Now in situ infrared spectroscopy and ab initio molecular dynamics are combined to elucidate the specific role of alkali cations and their trends.