This study explores the electrochemical reduction in CO2 using room temperature ionic liquids as solvents or electrolytes, which can minimize the environmental impact of CO2 emissions. To design effective CO2 electrochemical systems, it is crucial to identify intermediate surface species and reaction products in situ. The study investigates the electrochemical reduction in CO2 using a cobalt porphyrin molecular immobilized electrode in 1-n-butyl-3-methyl imidazolium tetrafluoroborate (BMI.BF4) room temperature ionic liquids, through in-situ surface-enhanced Raman spectroscopy (SERS) and electrochemical technique. The results show that the highest faradaic efficiency of CO produced from the electrochemical reduction in CO2 can reach 98%. With the potential getting more negative, the faradaic efficiency of CO decreases while H2 is produced as a competitive product. Besides, water protonates porphyrin macrocycle, producing pholorin as the key intermediate for the hydrogen evolution reaction, leading to the out-of-plane mode of the porphyrin molecule. Absorption of CO2 by the ionic liquids leads to the formation of BMI·CO2 adduct in BMI·BF4 solution, causing vibration modes at 1100, 1457, and 1509 cm-1. However, the key intermediate of CO2-· radical is not observed. The υ(CO) stretching mode of absorbed CO is affected by the electrochemical Stark effect, typical of CO chemisorbed on a top site.
Keywords: SERS; cobalt protoporphyrin; ionic liquid; pholorin.