Challenging a Deeply Buried Electrode By Vibrational Sum Frequency Spectroscopy. Towards the Understanding the CO2 Electroreduction on Ionic Liquid-Metal
Authors: Garciar-Rey Natalia , Dlott Dana D 
Published: February 2020

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.
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