Interrogating a Deeply Buried Electrode by Vibrational Sum Frequency Spectroscopy. Towards Understanding the Electroreduction at Ionic Liquid-Metal Interfaces

Understanding the molecular dynamics on buried electrodes is of significant interest in electrochemistry. There is a big gap of knowledge in the CO₂ electroreduction mechanism due to the limitations to access to the liquid-metal interface. Vibrational Sum Frequency Spectroscopy (VSFS) is a non-invasive and surface sensitive technique, with molecular level detection that can be used to probe electrochemical reactions occurring on the electrolyte-electrode interface [1]. In this study, we observed the CO₂ electroreduction to CO in ionic liquid on poly Ag using VSFS synchronized with cyclic voltammetry. In order to follow the CO₂ reaction in situ on the ionic liquid-Ag interface, the CO, CO₂ and imidazolium vibrational modes (resonant SFS) were monitored as a function of potential. We identified at which potential the CO was produced and how the EMIM⁺-BF₄^- played an important role in the electron transfer to the CO₂, lowering the intermediate, CO₂^- , energy barrier. We also present a new approach to reveal the double layer dynamics to the electrostatic environment by the study of the nonresonant SFS as a function of potential. In this analysis, we observed a strong third order effect in the susceptibility of the high electric field created in the double layer [2]. Finally, we discuss the intrinsic technical difficulties of probing deeply buried electrodes, as the IR absorption from the species of the electrolyte and how we tackled this problem.

Note: why do we specifically study this electrochemical cell? Ionic liquids (ILs) attracted a wide attention within the electrochemical community due to their unique properties; such a high charge density, stable electrolytes and low volatility. In particular, imidazolium-based ILs were proposed as a promising electrolyte to use in CO₂ fuel reactors, due to the CO₂ high solubility. These gas flow-electrochemical cells, convert the CO₂ into useful products such a CO, methane, ethanol [3]. The development of this technology can address two important environmental problems: the excess of CO₂ in the atmosphere and the use of a different energy sources than fossil fuels for transportation. However, CO₂ electroreduction is energetically very expensive, and higher energetic efficiency and reaction rates need to be fulfilled to become feasible [4]. Previous studies have shown, CO₂ electroreduction in a water mixture with imidazolium–based ILs on Ag nanoparticles at lower overpotential [5]. Our study help to understand the dynamics of the ionic liquid at electrified interfaces and the influence in the CO₂electroreduction to improve these gas electrochemical reactors.

[1] (a) Bain, C. D.; J. Chem. Soc., Faraday Trans., 1995, 91, 1281. (b) Tadjeddine, A.; Vidal, F.; In-situ Spectroscopic Studies of Adsorption at the Electrode and Electrocatalysis, Elsevier Science B.V., Amsterdam, 2007, pp. 273-298.

[2] (a) Eisenthal, K. B.; Chem. Rev.; 1996, 96 (4): 1343. (b) Koelsch, P.; Muglali, M. I.; Rohwerder, M. and Erbe, A.; J. Opt. Am. B, 2013, 30 (1), 219.

[3] Jhong, H.-R. M.; Ma, S.; Kennis, P. J. A.; Curr. Opin. Chem. Eng. 2013, 2: 191.

[4] Whipple, D. T. & Kenis, P. J. A.; J. Phys. Chem. Lett.; 2010, 1 (24), 3451.

[5] Rosen, B. A.; Salehi-Khojin, A.; Thorson, M. R.; Zhu, W.; Whipple, D. W., Kenis, P. J. A.; and Masel, R.I.; Science, 2011, 334 (6056), 643.