Room-temperature ionic liquids (IL) play an important role in lowering CO onset potentials for CO2 reduction reactions (CO2RR), but the reaction mechanism is still controversial. Using in operando IR absorption spectroscopy and in situ sum-frequency generation spectroscopy, we provide new information on the role of the cation for CO2RR. For this purpose, we have investigated CO2RR on polycrystalline Pt electrodes using 1-butyl-3-methylimidazolium [BMIM], 1-butyl-2,3-dimethylimidazolium [BMMIM], and 1-butyl-1-methylpyrrolidinium bis­(trifluoromethylsulfonyl)­imide [BMPyrr]­[NTf2] with 10 and 500 mM H2O. Cyclic voltammetry indicates CO2RR reduction activity for all three systems, with the highest current densities for [BMMIM]­[NTf2]. Spectroscopic investigation revealed a CO onset potential of −0.7 V vs. standard hydrogen electrode for [BMMIM/BMPyrr]­[NTf2], which is independent of the variation of the H2O concentration and where CO formation can occur via an electrostatically stabilized CO2 radical anion. On the other hand, [BMIM]­[NTf2] shows a strong dependence on H2O with an onset potential of −0.4 V at 500 mM H2O and the formation of an imidazolium-2-carboxylic acid as a reactive intermediate.

CO 2 reduction reactions (CO 2 RR) are interesting for power-to-x applications and have been studied on Pt electrodes in 1-ethyl-3-methylimidazolium dicyanamide [EMIM][DCA] as well as in 1ethyl-3-methylimidazolium [EMIM][BF 4 ], 1-butyl-3-methylimidazolium [BMIM][BF 4 ] and 1-ocytl-3-methylimidazolium tetrafluoroborate [OMIM] [BF 4 ] electrolytes. Cyclic voltammetry indicates a strong increase in activity for CO 2 RR with water concentration, which was investigated on a molecular level by using IR absorption spectroscopy (IRAS) and sum-frequency generation (SFG) to address both bulk and surface-adsorbed species. IRAS demonstrates that the formation of an imidazolium carboxylic acid intermediate occurs at electrode potentials as high as À 0.4 V, which depend on the choice of the RTIL and the water concentration. In addition, SFG spectroscopy provides evidence for the formation of CO on Pt atop sites and was used to determine the onset potential for the formation CO. In [BMIM] [BF 4 ] and [OMIM] [BF 4 ] electrolytes, the formation of CO is negligible even at very negative potentials of À 1.5 V, but for [EMIM][DCA] and [EMIM] [BF 4 ] the formation of CO is observed and the onset potential shifts significantly with the H 2 O concentration. Taking the Stark tuning rate (40 cm À 1 /V) of the CO band into account, we conclude that the CO coverage is fairly low, yet the presence of CO leads to deactivation of the Pt surface and to a decrease in reduction currents.

Room-temperature ionic liquids (ILs) have gained considerable attention as an important addition to conventional electrolytes because they exhibit large electrochemical windows and can reduce existing overpotentials in electrocatalysis. For the interfacial electrochemistry of ILs, a comprehensive understanding of molecular ions and the resulting electric double-layer structures as a function of electrode potential is mandatory, but the structures are largely different from conventional electrolytes. For that reason, we have studied the interfaces of Pt(111) in contact with ILs using 1-butyl-3-methylimidazolium[BMIM] and 1-butyl-2,3-dimethylimidazolium [BMMIM] cations as well as bis(trifluoromethylsulfonyl)imide [NTf 2 ] anions. We applied vibrational sumfrequency generation (SFG), where we interrogate vibrational bands from interfacial cations, anions, as well as interfacial water in situ and under potential control. Structuring of [NTf 2 ] anions and H 2 O with electrode potential show hysteresis while a strong Stark tuning was absent. This indicates that the IL ions are oriented in the vicinity of the interface, without being directly adsorbed to the Pt(111) surface. Using the C-H stretching band from CH groups at the imidazolium ring, the ring reorientation with electrode potential was qualitatively determined. The imidazolium ring reorients as a function of potential from a more parallel orientation to an upright orientation with respect to the interfacial plane. This leads to the formation of voids in the layered structure of ions at the interface, which can be then filled with H 2 O as evidenced by an increased SFG intensity from O-H stretching modes that are attributable to hydrogen-bonded interfacial water. Comparing the responses of the ILs, particularly of [BMMIM][NTf 2 ], shows a compact structure and a significantly pronounced rearrangement of the imidazolium ring that can also facilitates better incorporation of H 2 O and significantly affects the reorientation of [NTf 2 ] anions and, thus, causes a pronounced hysteresis with electrode potential.

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CO oxidation is fundamental to the development of new catalyst materials for fuel cells and key for complete oxidation of small alcohols like methanol or ethanol on Pt catalysts. So...