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

We report a combined computational and experimental work aimed at estimating the equilibrium potential for the electroreduction of CO₂ to CO₂^- (widely accepted to be a crucial and overpotential-determining step) and at providing an alternative view on the reason behind the lower overpotential for CO₂ reduction in imidazolium-based ionic liquid/water mixtures. To begin with, we obtained an 80 ps ab-initio molecular dynamics trajectory of the CO₂^- solvation structures in an 18% EMIM-BF₄/water mixture, which delivered no evidence of interaction between EMIM⁺ and CO₂^-. Next, using the Fc⁺/Fc couple as the non-aqueous reference, we calculated the equilibrium potential of the CO₂/CO₂^- couple in the mixture and aligned it with the aqueous SHE scale, proving that the equilibrium potential of CO₂/CO₂^- 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-BF₄. Finally, we compared the hydrogen bonding interactions between the CO₂^- radical and H₂O molecules in water and in the mixture, which showed that stabilization of CO₂^- by water molecules in the EMIM-BF₄/water mixture is stronger than in the aqueous medium. This suggests that water in EMIM-BF₄/water mixtures could be responsible for the low overpotential reported in these kinds of electrolytes.