Structural Transition in an Ionic Liquid Controls CO₂ Electrochemical Reduction

Broad-band multiplex vibrational sum-frequency generation spectroscopy (SFG) was used to study CO₂ reduction on a polycrystalline Ag electrode with a room-temperature ionic liquid (RTIL) electrolyte, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF₄), with 0.3 mol % water. The Ag/RTIL/H₂O system has been shown to reduce CO₂ with low overpotential and, depending on water concentration, with Faradaic efficiency of nearly 100% (Rosen, B. A.; Salehi-Khojin, A.; Thorson, M. R.; Zhu, W.; Whipple, D. T.; Kenis, P. J. A.; Masel, R. I. Science 2011, 334, 643-644). The adsorbed CO created by CO₂ reduction was probed with infrared (IR) pulses tuned to the CO stretch. Nonresonant (NR) SFG was used to probe the double layer. SFG showed that CO binds weakly to Ag at the CO₂ reduction threshold of -1.33 V (vs Ag/AgCl), so CO does not poison the surface. At potentials equal to or more negative than the threshold, the curvature of the parabolic potential-dependent NR intensity significantly increased, and the Stark shift of adsorbed CO, a measure of the surface field, more than doubled. The curvature increase indicates a potential-driven structural transition in the RTIL within the double layer. This transition was a property of the RTIL itself since it occurred whether or not CO₂ was present. Significantly, the RTIL transition and the increased surface field occurred precisely at the CO₂ reduction threshold. Thus, we have demonstrated a close association between an electrochemically driven structural transition of the RTIL and low overpotential CO₂ reduction.