Water-In-Salt Environment Reduces the Overpotential for Reduction of CO₂ to CO₂^– in Ionic Liquid/Water Mixtures

Abstract

We report a combined computational and experimental work aimed at estimating the equilibrium potential for the electroreduction of CO 2 to CO 2 − (widely accepted to be a crucial and overpotential-determining step) and at providing an alternative view on the reason behind the lower overpotential for CO 2 reduction in imidazolium-based ionic liquid/water mixtures. To begin with, we obtained an 80 ps ab-initio molecular dynamics trajectory of the CO 2 − solvation structures in an 18% EMIM−BF 4 / water mixture, which delivered no evidence of interaction between EMIM + and CO 2 − . Next, using the Fc + /Fc couple as the nonaqueous reference, we calculated the equilibrium potential of the CO 2 /CO 2 − couple in the mixture and aligned it with the aqueous SHE scale, proving that the equilibrium potential of CO 2 /CO 2 − in the mixture is about 0.3 V less negative than in the aqueous medium. We then looked for the origin of this catalytic effect by comparing the computed vibrational spectra with experimental Fourier transform infrared spectra. This revealed the presence of two water populations in the mixture, namely, bulk-like water and water in the vicinity of EMIM−BF 4 . Finally, we compared the hydrogen bonding interactions between the CO 2 − radical and H 2 O molecules in water and in the mixture, which showed that stabilization of CO 2 − by water molecules in the EMIM−BF 4 /water mixture is stronger than in the aqueous medium. This suggests that water in EMIM−BF 4 /water mixtures could be responsible for the low overpotential reported in these kinds of electrolytes.

Tailoring the Interfacial Water Structure by Electrolyte Engineering for Selective Electrocatalytic Reduction of Carbon Dioxide

“…A band at 3610 cm –1 has been observed in the bulk of water–ionic liquid mixtures and was attributed to water molecules with a low degree of hydrogen bonding within a water-in-salt environment. It was suggested later that these water molecules contribute to stabilizing the CO 2 – radical formed after the first electron transfer to CO 2 in the CO2RR, thereby leading to enhanced activity for the CO2RR in these ionic liquid–water mixtures . It is reasonable to also attribute the OH-stretching contribution at 3600 cm –1 to interfacial water molecules with a low degree of hydrogen bonding within a DMF-rich environment, although we have no evidence in this case of enhanced activity for the CO2RR induced by these water molecules.…”

“…Ionic additives including organic salts, surfactants, − and ionic liquids − have been used to enhance the CO2RR in aqueous electrolytes. Neubauer et al studied the effect of the concentration of the ionic liquid (IL) 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM]­[OTF]) (0–50 mol %) in 10 mM KHCO 3 on the CO2RR on silver.…”

Enhancing C≥2 product selectivity in electrochemical CO2 reduction by controlling the microstructure of gas diffusion electrodes

“…Several different approaches in this direction have been recently assessed, from the development of engineered pathways for CO 2 and H + /OH À transport, 37 to the insertion of hydrophobic zones near the active sites, 23 to the use of complex electrolyte solutions. 30,38 As these approaches have yielded important -but not sufficient -selectivity improvements, we expect that a higher degree of control of the local catalyst environment, for example achieved by the targeted introduction of hydrophobic -hydrophilic zones within the GDE, may be necessary to obtain a single C Z2 product during CO 2 RR.…”

Exploring how cation entropy influences electric double layer formation and electrochemical reactivity

“…We selected ionic liquid-mediated CO 2 reduction to CO on Ag electrodes as a model system, as ionic liquid molecular structures can be systematically modulated via synthetic protocols. 36 Ionic liquids are salts that are often composed of weakly coordinating organic cations and anions and have been heavily studied as excellent promoters of CO 2 reduction in both aqueous [37][38][39][40] and non-aqueous electrolytes. 30,41,42 Imidazoliumbased salts are the prototypical class of ionic liquids used for mediating CO 2 electroreduction due to their propensity to donate protons and coordinate CO 2 .…”