Cations have been widely reported to significantly affect the activity and selectivity of electrocatalytic CO2 reduction, but the mechanism remains the topic of debate. Theoretical calculations predict larger cations induce stronger local electric fields, which foster CO2 adsorption and stabilize intermediates, while another theory proposes that the stronger buffering capacity of larger cations helps to maintain a higher interfacial CO2 concentration. In this study, we try to reconcile these theories by studying fields at the active sites and taking the Onsager reaction field into account. We employ vibrational sum frequency generation (VSFG) spectroscopy to study the local electric field at the electrode-electrolyte interface by monitoring in-situ generated CO on Au. The in-situ produced CO serves as a Stark reporter enabling measurement of the total electric field at active sites. The Stern layer electric field and Onsager reaction field are then disentangled from the total field. Contrary to many theoretical reports, we show that instead of the Stern layer electric field, the interfacial Onsager reaction field directly correlates with the catalytic performance, pointing to a direct relationship between interfacial solvation structure and CO2 reduction reaction kinetics. In this series of cation-dependent measurements, the Cs+ electrolyte behaves as an outlier in terms of both the measured interfacial electric field and catalytic performance, suggesting partial desolvation of this cation upon adsorption. These findings are supported by cation-dependent measurements of the interfacial water spectrum under reaction conditions. These findings indicate that the interfacial solvation structure and its associated Onsager reaction field cannot be ignored when modeling CO2 reduction reaction kinetics.

Mechanism of specific cation effects on CO2 reduction on gold: A new perspective from monitoring in-situ generated CO using plasmon-enhanced SFG spectroscopy

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.