In Situ Spectroscopic Characterization of an Intermediate of CO₂ Electroreduction on a Au Electrode in Room-Temperature Ionic Liquids

Abstract

Room-temperature ionic liquids (RTILs) are fascinating electrolytes for CO 2 electroreduction owing to their low overpotentials. Although it is well accepted that the key to understanding the low overpotential is the stabilization of the intermediate, its mechanism is still under discussion. To elucidate the intermediate stabilization mechanism, RTIL/electrode interfaces under CO 2 reduction were observed using surface-enhanced infrared absorption spectroscopy (SEIRAS). This in situ interfaceselective spectroscopy allows us to detect CO 2•− that is not observed in aqueous solutions. For a negative-going potential scan, identical onsets were observed for increasing SEIRA band intensities of CO 2•− and RTIL cations and reduction current. This result indicates that RTIL cations stabilize CO 2 •− on the electrode, and this stabilizing effect can be ascribed to the origin of the low overpotentials of CO 2 electroreduction in RTILs.

Mechanism of CO2 Capture and Release on Redox-Active Organic Electrodes

“…In ATR–SEIRAS using PAQ under CO 2 , the IR peak assignable to the asymmetric stretching vibration of CO 2 , ν as (CO 2 ), was observed at 2200 cm –1 , which was estimated to have been caused by the CO 2 molecule correlated with AQ by isotope experiments. Previous studies have indicated that the lower wavenumber in comparison to that of free CO 2 (2396 cm –1 ) is due to adsorption and that the shift in wavenumber of CO 2 by physical adsorption is less than the shift caused by chemisorption. ,, Especially, Motobayashi et al have studied the shift of the peak attributable to CO 2 as a result of its strong adsorption on the Au film at a higher negative potential of −2.6 V versus Fc/Fc + , which is similar to the present study of ATR–SEIRAS measurements in aprotic conditions under CO 2 . On the basis of the results of ATR–SEIRAS measurements and DFT calculations, the peak at 2200 cm –1 is presumed to be CO 2 physisorbed on the AQ unit of PAQ, although there is no clear evidence.…”

“…52,61,62 Especially, Motobayashi et al have studied the shift of the peak attributable to CO 2 as a result of its strong adsorption on the Au film at a higher negative potential of −2.6 V versus Fc/Fc + , which is similar to the present study of ATR−SEIRAS measurements in aprotic conditions under CO 2 . 52 On the basis of the results of ATR− SEIRAS measurements and DFT calculations, the peak at 2200 cm −1 is presumed to be CO 2 physisorbed on the AQ unit of PAQ, although there is no clear evidence. In addition, the peak at 2200 cm −1 is very weak, which correlates well with small ΔG of −7.3 kJ/mol for AQ−(CO 2 − ) 2 .…”

“…The reasons of the assumption are follows: (1) ionic liquid employed for ATR–SEIRAS measurements did not contain water as the proton donor for CO 2 reduction, and (2) Stark shift was not observed in our measurements, which is generally discerned as the peak shift in dependence upon the applied potential and especially known to be prominent in the shift of the CO peak. − , In addition, Motobayashi et al also showed that the detection of CO by the ATR–SEIRAS measurement in aprotic ionic liquid is limited. That is, the peak being ascribed to CO on Au, which occurred by the disproportional reaction of CO 2 or reduction of CO 2 with residual H 2 O, is observed only at a higher negative potential of −2.6 V versus Fc/Fc + (about −2.5 V versus Ag/AgNO 3 ) . A more detailed discussion will be given in the section describing the DFT calculations.…”

“…That is, the peak being ascribed to CO on Au, which occurred by the disproportional reaction of CO 2 or reduction of CO 2 with residual H 2 O, is observed only at a higher negative potential of −2.6 V versus Fc/Fc + (about −2.5 V versus Ag/AgNO 3 ). 52 A more detailed discussion will be given in the section describing the DFT calculations. The results of measurements performed under 12 CO 2 , 13 CO 2 , and N 2 by scanning from a negative potential to a positive potential are shown in panels b and d of Figure 3 and Figure S12b of the Supporting Information, respectively.…”

Understanding the role of imidazolium-based ionic liquids in the electrochemical CO2 reduction reaction

“…Another advantage of RTILs is their ability to stabilize EC CO 2 RR reaction intermediates, such as the CO 2 •- anion radical that is the rate-determining step (RDS) of the reaction at high current densities 27 . For instance, Motobayashi et al 28 observed RTIL cations stabilizing the CO 2 •- radical intermediate by using surface-enhanced infrared absorption spectroscopy (SEIRAS). Ratschmeier et al 29 also demonstrated the CO formation via an electrostatically stabilized CO 2 •- radical by the RTILs cation by using in operando IR absorption spectroscopy and in situ sum-frequency generation spectroscopy.…”