| Summary: | This study explores the electrochemical reduction in CO<sub>2</sub> using room temperature ionic liquids as solvents or electrolytes, which can minimize the environmental impact of CO<sub>2</sub> emissions. To design effective CO<sub>2</sub> electrochemical systems, it is crucial to identify intermediate surface species and reaction products in situ. The study investigates the electrochemical reduction in CO<sub>2</sub> using a cobalt porphyrin molecular immobilized electrode in 1-<i>n</i>-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 CO<sub>2</sub> can reach 98%. With the potential getting more negative, the faradaic efficiency of CO decreases while H<sub>2</sub> 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 CO<sub>2</sub> by the ionic liquids leads to the formation of BMI·CO<sub>2</sub> adduct in BMI·BF<sub>4</sub> solution, causing vibration modes at 1100, 1457, and 1509 cm<sup>−1</sup>. However, the key intermediate of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>CO</mi></mrow><mn>2</mn><mo>−</mo></msubsup><mo>·</mo></mrow></semantics></math></inline-formula> radical is not observed. The <i>υ</i>(CO) stretching mode of absorbed CO is affected by the electrochemical Stark effect, typical of CO chemisorbed on a top site.
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